Dark matter, the universe’s most famous invisible houseguest, is believed to make up most of the matter in the cosmos - yet no one can see it, touch it, or get it to RSVP. Unlike ordinary matter, it refuses to interact with light or electromagnetic forces, leaving gravity as the only known way to detect its presence. Now, researchers think colliding black holes might finally give this elusive substance a reason to show itself.
Physicists at MIT and several European institutions have developed a method to spot possible dark matter signals hidden inside gravitational waves - those ripples in spacetime created when massive objects like black holes spiral together and merge. If those black holes travel through dense clouds of dark matter before colliding, the resulting gravitational waves could carry subtle traces of that interaction, like a cosmic handprint on a window. The team tested their approach using publicly available data from LIGO-Virgo-KAGRA (LVK), the international network of gravitational wave observatories that monitors black hole mergers and other distant cosmic events.
The researchers analyzed signals from LVK’s first three observing runs, focusing on 28 of the clearest gravitational wave events detected so far. For 27 of those events, the signals matched what scientists would expect from black holes merging in empty space - business as usual in the vacuum. But one signal, known as GW190728, appeared different. According to the team’s analysis, the pattern of that gravitational wave may contain evidence of an interaction with dark matter. The researchers stress this does not count as a confirmed discovery - more like a promising lead in a cosmic cold case.
“We know that dark matter is around us. It just has to be dense enough for us to see its effects,” says Josu Aurrekoetxea, a postdoc in the MIT Department of Physics. “Black holes provide a mechanism to enhance this density, which we can now search for by analyzing the gravitational waves emitted when they merge.” The findings appear in Physical Review Letters, co-authored by Aurrekoetxea, LVK member Soumen Roy of Université Catholique de Louvain (UCLouvain) in Belgium, Rodrigo Vicente of the University of Amsterdam, Katy Clough of Queen Mary University of London, and Pedro Ferreira of Oxford University.
Dark matter remains one of physics’ biggest embarrassments - scientists infer its existence because gravity around galaxies appears stronger than visible matter alone can explain, and observations of gravitational lensing show an extra mass bending light. Current estimates suggest dark matter could account for more than 85 percent of the matter in the universe, but researchers still don’t know what it actually is. One proposed form involves extremely lightweight particles called “light scalar” particles, which theories suggest can behave like coordinated waves near black holes. Scientists believe that when these waves encounter a rapidly spinning black hole, the black hole’s rotational energy can transfer into the dark matter waves, dramatically increasing their density - a process known as superradiance, which has been compared to whipping cream into butter. (We’re not sure dark matter tastes like butter, but the analogy holds.) If the density becomes high enough, the dark matter could alter the gravitational waves produced when black holes collide.
To investigate this, the researchers built detailed simulations of black hole mergers under many different conditions, varying factors like the masses and sizes of the black holes, the amount of surrounding dark matter, and its density. Using those simulations, the team predicted how gravitational waves would appear if black holes merged inside a dense dark matter environment rather than in a vacuum, and accounted for how those waves would change as they traveled across millions of light years to Earth. Comparing their predictions with actual LVK observations, GW190728 was the only event out of 28 that showed agreement with the dark matter scenario. GW190728 was first detected on July 28, 2019; earlier studies determined the signal came from two black holes with a combined mass about 20 times that of the sun. According to the new analysis, those black holes may have merged within a dense cloud of dark matter.
“The statistical significance of this is not high enough to claim a detection of dark matter, and further checks should be performed by independent groups,” Aurrekoetxea says. “What we think is important to highlight is that without waveform models like ours, we could be detecting black hole mergers in dark matter environments, but systematically classifying them as having occurred in vacuum.” Researchers say the growing number of gravitational wave observations could make this approach increasingly useful. “We now have the potential to discover dark matter around black holes as the LVK detectors keep collecting data in the coming years,” says co-author Soumen Roy, who led the data analysis. “It is an exciting time to search for new physics using gravitational waves.” Adds co-author Rodrigo Vicente, who developed the analytical model of the signal: “Using black holes to look for dark matter would be fantastic. We would be able to probe dark matter at scales much smaller than ever before.” The research was supported in part by the U.S. National Science Foundation and MIT’s Center for Theoretical Physics - a Leinweber Institute.