Dark Matter Depletion Channels as Technosignatures:

A Proposed Search Strategy Using Weak Gravitational Lensing Surveys

old preprint please ignore

We propose a novel class of technosignature: linear or arc-shaped dark matter depletion channels in galactic halos, potentially produced by hypothetical faster-than-light transit mechanisms that interact with the dark matter eld. Regardless of the specic propulsion mechanism, any FTL technology that scoops, displaces, or otherwise disturbs dark matter during transit would carve persistent corridors of reduced dark matter density. Because dark matter is collisionless, these channels would not rell on astrophysically short timescales, persisting for 10 6 to 10 8 years. We argue that such features are detectable via weak gravitational lensing in current and forthcoming survey data, including DES Y3, Euclid, and Rubin Observatory/LSST. We describe the expected morphology, propose a detection pipeline based on the Hough transform applied to convergence maps, and outline criteria for distinguishing articial depletion channels from natural structures such as cosmic laments, tidal streams, and survey artifacts. This proposal is testable with existing public data and requires no assumption about the validity of any specic FTL mechanism.

1. Introduction The search for extraterrestrial intelligence (SETI) has expanded considerably beyond radio signal detection. Modern technosignature research encompasses infrared excess from Dyson spheres (Wright et al. 2014), industrial atmospheric pollutants on exoplanets (Lin et al. 2014), and megastructure transit signatures (Boyajian et al. 2016). However, one category of observable has received almost no attention: signatures imprinted on the dark matter distribution by advanced technological activity. Dark matter constitutes approximately 27% of the mass-energy content of the universe and dominates the gravitational potential of galactic halos. Any technology that interacts with dark matter during operation would leave traces in the dark matter density eld. Because dark matter is collisionless and dissipationless, these traces would persist far longer than signatures in baryonic matter, making them uniquely durable technosignatures.In this paper, we consider a specic class of hypothetical technosignature: linear or arc-shaped channels of dark matter depletion in galactic halos, such as might be produced by faster-than-light transit technologies that interact with the ambient dark matter eld. We emphasize that this proposal does not depend on any particular FTL mechanism being physically realizable. Rather, we ask: if such technologies exist, what observable signatures would they leave, and can we search for those signatures with existing instruments? We show that the expected morphology of such features is distinct from known astrophysical structures, that detection is feasible with current and near-future weak gravitational lensing surveys, and that a concrete detection pipeline can be constructed using established computational techniques.
2. The Observable Signature 2.1 Physical Model We consider a hypothetical transit technology that displaces dark matter along its trajectory. The specic interaction mechanism is left unspecied; we require only that the technology removes or redistributes dark matter from a corridor of characteristic width w along a path of length L through a galactic halo. By conservation of mass, displaced dark matter accumulates along the corridor boundaries, forming density ridges anking the depletion channel. The resulting structure has three components: (1) a central depletion channel of reduced dark matter density, (2) anking density ridges where displaced matter accumulates, and (3) a characteristic geometry determined by the transit path. We argue that optimal transit paths would arc through the galactic halo rather than passing through the disk, because the halo contains the highest integrated dark matter column density along most sightlines and avoids baryonic gas drag within the disk. Although civilization advanced enough may ignore these trade os if there is a lower limit on speed, as traveling through the disk is signicantly faster than exiting the disk and traveling through the halo. 2.2 Persistence A critical property of dark matter depletion channels is their longevity. Unlike disturbances in baryonic gas, which are erased by pressure forces on timescales of millions of years, dark matter is collisionless. There is no pressure gradient to rell a void in the dark matter distribution. The only mechanism for erasure is gravitational relaxation, which operates on timescales comparable to the dynamical time of the halo (approximately 10 8 to 10 9 years for Milky Way-scale halos). Individual depletion channels would therefore persist for geologicallysignicant timescales, and heavily-tracked corridors would accumulate deeper depletion over time. 2.3 Cumulative Eects A single transit would produce a negligibly small density perturbation. However, a sustained transportation network operating over megayear timescales would produce cumulative depletion along frequently-used routes. For a corridor traversed N times, each transit removing a fraction f of the dark matter within the corridor cross-section, the remaining density fraction is (1 − f) N . For plausible parameters (f ~ 10 −2 , N ~ 10 5 to 10 8 ), signicant depletion (>1%) is achievable and would produce measurable lensing signatures.
3. Detection Strategy 3.1 Weak Gravitational Lensing Weak gravitational lensing provides a direct probe of the total matter distribution, including dark matter, without requiring any electromagnetic emission from the target structure. Convergence maps reconstructed from the coherent distortion of background galaxy shapes yield projected mass density maps in which dark matter depletion channels would appear as linear or arc-shaped features of negative convergence (underdensity), convergence (overdensity) ridges. anked by positive Current weak lensing surveys provide sucient sky coverage and depth to conduct a meaningful search. The Dark Energy Survey Year 3 (DES Y3) convergence maps cover approximately 4,100 square degrees (Jerey et al. 2021). The combined DECADE+DES Y3 mass map extends this to 13,000 square degrees using 270 million galaxies (Gatti et al. 2025) and has already demonstrated the ability to detect cosmic laments directly from the convergence eld. The forthcoming Euclid survey will map approximately 14,000 square degrees with substantially greater depth, measuring shapes of approximately 1.5 billion galaxies (Euclid Collaboration, Mellier et al. 2025). 3.2 Detection Pipeline We propose a multi-stage detection pipeline. First, apply a spatial high-pass lter to the convergence map to remove structures larger than approximately 1 degree, isolating features at the angular scales relevant to individual halo structures. Second, apply the Hough transform, a standard line-detection algorithm, to the ltered convergence map, operating on pixels with signicantly negative convergence values. The Hough transform accumulates votes in parameter space (angle, oset), with peaks corresponding to linear features. For arc-shapedfeatures, a generalized Hough or Radon transform can detect curved paths. Third, for each candidate detection, extract the perpendicular convergence prole and test for the wake signature: a negative central channel anked by positive ridges. Fourth, cross-reference candidates against catalogs of known structures, including cosmic laments, tidal streams, and survey artifacts, to identify unexplained residuals. 3.3 Cross-Wavelength Conrmation Candidate detections from lensing should be cross-referenced with X-ray survey data. If baryonic gas exists along the wake path, shock compression at the density ridges could produce faint soft X-ray emission. The eROSITA all-sky survey (Merloni et al. 2024), with approximately 900,000 X-ray sources and publicly available event les, provides a suitable dataset for this purpose. A lensing-detected dark matter void anked by enhanced X-ray emission, with no associated galaxy cluster or known astrophysical source, would constitute a strong candidate.
4. Distinguishing Articial from Natural Features Several natural astrophysical structures produce linear or lamentary features in mass maps. The key discriminants between articial depletion channels and natural structures are as follows. Geometry. Cosmic laments are overdense structures connecting galaxy clusters, following the gravitational potential of the cosmic web. They are broad (typically several megaparsecs), curved by gravity, and always associated with overdensity. Depletion channels would be narrow (sub-megaparsec), geometrically smooth (straight lines or regular arcs rather than gravitationally-curved paths), and dened by underdensity. Tidal streams from disrupted satellite galaxies follow orbital trajectories and are always associated with a progenitor; depletion channels would lack any progenitor. Flanking ridges. Natural underdensities in the dark matter eld (cosmic voids) are not anked by narrow overdense ridges along their boundaries. The presence of symmetric, narrow density ridges immediately anking a linear underdensity would be inconsistent with any known natural process and would be a strong indicator of mass displacement by an external agent. Location. Depletion channels would preferentially occur within galactic halos, potentially connecting high-mass stellar concentrations (as expected for transportation routes between population centers). Natural dark matter underdensities (voids) occur between halos, not within them.Multiplicity. A transportation network would produce multiple channels within the same halo, potentially converging on common endpoints. The presence of several geometrically regular underdensities within a single halo, particularly if they intersect at discrete points, would be extremely dicult to explain through natural processes.
5. Expected Sensitivity The detectability of a depletion channel depends on the fractional density reduction, the channel width and length, and the noise properties of the lensing survey. For current DES Y3 data, the convergence noise per resolution element is approximately κ ~ 0.01, dominated by galaxy shape noise. Averaging along a linear feature of length N resolution elements reduces the eective noise by a factor of N1/2 . A 10 kpc channel spanning approximately 100 resolution elements at moderate redshift yields an eective noise of κ ~ 10 −3 . A channel with a fractional dark matter depletion of 1% along a 10 kpc path produces a convergence signal of approximately κ ~ 10 −6 , well below the detection threshold. However, a heavily-tracked route with 10% or greater depletion, or a channel observed in a nearby galaxy where angular resolution is more favorable, could approach κ ~ 10 −4 to 10 −3 , placing it within reach of Euclid and Rubin/LSST survey depths. The most favorable targets are edge-on spiral galaxies within approximately 100 Mpc, where individual halo substructure can be resolved.
6. Discussion This proposal occupies an unusual position in technosignature research: it requires no assumption about the validity of FTL travel or any specic propulsion mechanism. The argument is conditional. If any technology exists that displaces dark matter during transit, then depletion channels are the expected observable consequence, and weak gravitational lensing is the appropriate detection method. The value of the proposal lies in identifying a previously unconsidered observable and connecting it to existing survey capabilities. The collisionless nature of dark matter makes it a uniquely persistent recording medium for large-scale technological activity. While electromagnetic technosignatures fade on timescales of thousands to millions of years, dark matter depletion channels would persist for hundreds of millions of years, vastly expanding the temporal window for detection. A civilization that ceased FTL transit millions of years ago could still leave detectable traces in the dark matter distribution today. The primary limitation of this search is sensitivity. Current surveys are unlikely to detect depletion channels from anything less than a heavily-utilized transportation network sustained over megayear timescales. However, this is consistent with the general principle in SETI that themost detectable signatures come from the most resource-intensive activities of the most advanced civilizations. The forthcoming generation of lensing surveys, particularly Euclid (rst major data release October 2026) and Rubin/LSST, will improve sensitivity by approximately an order of magnitude.
7. Conclusion We have proposed dark matter depletion channels as a novel class of technosignature and outlined a concrete search strategy using existing and forthcoming weak gravitational lensing surveys. The detection pipeline, based on the Hough transform applied to convergence maps with anking-ridge verication, can be implemented using established computational tools. The DECADE+DES Y3 mass map, with its demonstrated capability for detecting linear features in the dark matter distribution, provides an immediately available dataset for an initial search. Euclid and Rubin/LSST will substantially improve sensitivity within the next two to ve years. Regardless of whether this search yields positive results, it represents a low-cost extension of existing survey science into a previously unexplored region of technosignature parameter space. The computational tools already exist, the data is public or forthcoming, and the search can be conducted as a secondary analysis of surveys designed for cosmological research. We encourage the community to consider dark matter substructure as a potential repository of technosignature information. References Boyajian, T. S. et al., 2016, MNRAS, 457, 3988 Euclid Collaboration: Mellier, Y. et al., 2025, A&A, in press Gatti, M. et al., 2025, arXiv:2509.03798 (DECADE+DES Y3 mass map) Jerey, N. et al., 2021, MNRAS, 505, 4626 (DES Y3 mass maps) Lin, H. W., Gonzalez Abad, G., & Loeb, A., 2014, ApJ, 792, L7 Merloni, A. et al., 2024, A&A, 682, A34 (eROSITA eRASS1) Wright, J. T. et al., 2014, ApJ, 792, 26