Although it makes up about 85 percent of all matter in the universe, dark matter is frustratingly hard to pin down. In order to figure out what it is, much of the search is about ruling out what dark matter isn't, and now physicists at Stanford and the SLAC National Accelerator Laboratory have narrowed it down further. Using observations of galaxies orbiting the Milky Way, the team found that dark matter is likely lighter than previously thought – and interacts even less with normal matter.
Dark matter is so called because it doesn't emit or react to light in any way, and rarely interacts with regular matter, making it all but invisible to us. The only way we know anything is there at all is because of its gravitational effects on the stuff we can see.
What dark matter actually is and what its properties are remains open for debate, but scientists have developed a few models. One of the most widely-accepted is known as the Lambda cold dark matter (Lambda CDM) model, and it assumes a few things about the mysterious stuff: it's "cold" meaning it moves much slower than the speed of light, and it's "collisionless," so it very rarely bumps into regular matter.
The Lambda CDM model also tells us that dark matter played a vital role in the formation of the universe as we know it. In the early days, the cosmos was basically flat and uniform, but because it moved slower, dark matter tended to clump together in "haloes." Eventually the gravity drew regular matter into those clumps, causing galaxies to form in structures that look like giant sparkling spider webs.
While this model has worked well to explain these large-scale structures, it tends to fall apart when applied to smaller scales, like individual galaxies. For example, the model predicts that our own Milky Way should be orbited by thousands of smaller satellite galaxies, but as of right now only 59 have been discovered.
The researchers on the new study used this as a starting point to tweak the model. They ran simulations of how the universe looked when filled with dark matter of varying properties, and then overlaid the resulting dark matter haloes with the known structure of the Milky way's satellite galaxies, to see how well it all fit.
They found that for everything to fit together neatly, dark matter must have a lighter mass and must be "warmer" (i.e. moves a bit faster) than previously assumed. It also seems to interact even less often with regular matter – about a thousand times more weakly than the previous limit. That might explain why none of the many experiments designed to detect those interactions have registered any signals yet.
"What's really exciting is that our study nicely bridges experimental observations of faint galaxies today with theories of dark matter and its behavior in the early universe," says Ethan Nadler, lead author of the study. "It connects a lot of pieces, and by doing so it tells us something very profound about dark matter. Although we still don't know what dark matter is made of, our results are a step forward that sets tighter limits on what it actually can be."
The research was published in the Astrophysical Journal Letters. The simulated evolution of dark matter can be seen in the video below.
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