We don't know what dark matter is, but that doesn't stop astronomers from using it to their advantage. Dark matter is part of what makes gravitational lensing so effective. Astronomers expect the Roman Space Telescope to find 160,000 gravitational lenses, and dark matter makes a crucial contribution to these lenses.
Not only is dark matter part of these gravitational lenses, but in an elegant twist, the lenses themselves will be used to study dark matter. Gravitational lensing relies on massive galaxy clusters in the foreground to magnify extremely distant objects in the background. Since dark matter makes up 85% of a galaxy cluster's matter, even more in some cases, gravitational lensing would be far less effective without it. Merely by using dark matter as a gravitational lens, scientists can learn more about it.
New research in The Astrophysical Journal calculates that the upcoming Nancy Grace Roman Space Telescope will detect about 160,000 strong gravitational lenses. It's titled "The Roman View of Strong Gravitational Lenses," and the lead author is Bryce Wedig. Wedig is a physics graduate at Washington University in St. Louis.
A strong gravitational lens is one where all three elements are in precise alignment: the observer, the gravitational lens, and the background object. These lenses have more dramatic magnification than weaker lenses. Though the Roman Telescope is expected to detect 160,000 of them, the researchers think that about 500 of them will be precise enough that astronomers can use them to study the structure of dark matter at small scales.
"Galaxy–galaxy strong gravitational lenses can constrain dark matter models and the Lambda cold dark matter cosmological paradigm at subgalactic scales," the researchers write. Currently, astronomers face a lack of strong lenses with the high signal-to-noise ratio (SNR) and angular resolution needed to see at these scales. But the Roman Telescope will change that. "With its remarkable 0.281 square degree field of view and diffraction-limited angular resolution of ~0".1, Roman is uniquely suited to characterizing dark matter substructure from a robust population of strong lenses," the authors explain.
“The current sample size of these objects from other telescopes is fairly small because we’re relying on two galaxies to be lined up nearly perfectly along our line of sight,” Wedig explained in a press release. “Other telescopes are either limited to a smaller field of view or less precise observations, making gravitational lenses harder to detect.”
This illustration shows the galaxy–galaxy strong-lensing configuration. Light from a distant source galaxy is gravitationally lensed by the foreground lensing galaxy, forming multiple distorted and magnified images of the source. The lensing potential is due to a main halo and a population of subhalos in the lens plane. Image Credit: Wedig et al. 2025 The Astrophysical Journal.
It's all related to the Roman's High Latitude Wide Area Survey (HLWAS), one of three core surveys the telescope will undertake. In about 1.5 years, the Roman will image 12% of the entire sky with filters and spectroscopy to reveal more than one billion galaxies. The new research is based on simulations of the HLWAS that show how it will find strong gravitational lenses. "We simulate a population of galaxy–galaxy strong lenses across cosmic time with cold dark matter subhalo populations, select those detectable in the HLWAS, and generate simulated images accounting for realistic Wide Field Instrument detector effects," the authors write, explaining how they arrived at the 500 strong lenses.
Those 500 lenses will be used to study dark matter on a smaller scale. The dark matter under investigation won't be in the magnified galaxies; instead, it'll be in the strong gravitational lens itself.
“Roman will not only significantly increase our sample size — its sharp, high-resolution images will also allow us to discover gravitational lenses that appear smaller on the sky,” said study co-author Tansu Daylan, principal investigator of the science team conducting this research program and an assistant professor of physics in Arts & Sciences at Washington University. “Ultimately, both the alignment and the brightness of the background galaxies need to meet a certain threshold so we can characterize the dark matter within the foreground galaxies.”
This photo shows the Optical Telescope Assembly for NASA’s Nancy Grace Roman Space Telescope in a clean room at the agency’s Goddard Space Flight Center. It's scheduled to launch in October 2026. Image Credit: NASA/Chris Gunn
These detailed observations will help address some problems with the Lambda Cold Dark Matter theory. It's the most widely-accepted model of Big Bang cosmology. It does a good job of describing the large scale Universe, but breaks down on smaller scales.
One of the things Lambda CDM predicts is many more dwarf galaxies around larger galaxies like the Milky Way than we can observe. Detailed supercomputer simulations show there should be thousands more of these galaxies and their small dark matter haloes. The Roman's 500 strong lenses should be able to detect smaller amounts of light being bent by these small haloes. If they're detected, then our Lambda CDM model becomes that much stronger.
“Finding gravitational lenses and being able to detect clumps of dark matter in them is a game of tiny odds. With Roman, we can cast a wide net and expect to get lucky often,” Wedig said. “We won’t see dark matter in the images — it’s invisible — but we can measure its effects.”
The dark matter issue comes down to one overarching question: What type of particle is it? Some candidates are WIMPs (Weakly Interacting Massive Particles), Axions, and Sterile Neutrinos.
“Ultimately, the question we’re trying to address is: What particle or particles constitute dark matter?” Daylan added. “While some properties of dark matter are known, we essentially have no idea what makes up dark matter. Roman will help us to distinguish how dark matter is distributed on small scales and, hence, its particle nature.”
Detecting individual small dark matter subhaloes is critical to understanding CDM. "Single-subhalo detections have been used to place constraints on WDM (Warm Dark Matter), the CDM substructure mass fraction and subhalo density profiles," the authors write in their paper. Warm Dark Matter is a simple modification of CDM where the dark matter particles have an initial velocity.
The authors say that the Roman's HLWAS won't have the ability to detect single subhaloes with less than 10 billion Earth masses. But another of its core surveys, the High-Latitude Time-Domain Survey (HLTDS) may be able to. The details of that survey haven't been finalized yet.
Prior to the Roman's launch, the researchers will look make use of data from other telescopes like the Vera Rubin Observatory and the Euclid Space Telescope. “Once Roman’s images are in hand, the researchers will combine them with complementary visible light images from Euclid, Rubin and Hubble to maximize what’s known about these galaxies,” said Simon Birrer, an assistant professor at Stony Brook University and a co-investigator of the research program.
“We will push the limits of what we can observe, and use every gravitational lens we detect with Roman to pin down the particle nature of dark matter,” Daylan said.
To crack the code of dark matter, researchers need to place tighter constraints on the microphysics of dark matter. While scientists have been able to observe dark matter's effect on large-scale structures, a deeper understanding of it on the particle level eludes them. The Roman Space Telescope and its 500 strong gravitational lenses should help.
