Looking up at the night sky is an amazing way to get in touch with the universe, but even though we can see stars, nebula and other galaxies, the majority of the universe is made up of material we can’t see. Dubbed ‘dark matter,’ this unobservable material makes up nearly 80 percent of the universe — it is, quite literally, the glue that holds the universe together. Scientists have spotted galaxies made almost entirely of dark matter, such as the galaxy dubbed Dragonfly 44.
What is dark matter, and why is it so surprising scientists may have found a galaxy that completely lacks this elusive material?
Think back to your middle school science classes where you learned about neutrons, electrons and protons. These three types of particles are dubbed baryonic matter and make up the visible materials that we are familiar with.
Dark matter is most likely made up of non-baryonic matter — particles that don’t interact with the neutrons, electrons and protons in any normal way. That lack of interaction makes these particles nearly impossible to detect — we only know it’s there because we can observe the movement of stars. There is some form of matter out there — we just can’t see it yet.
If dark matter is the scaffolding that holds the universe together, then NGC 1052-DF1 is missing its glue. This galaxy, recently observed by Pieter van Dokkum using the Dragonfly Telephoto Array, seems to have no dark matter at all.
This galaxy is what is known as an ultra-diffuse galaxy — it is nearly as large as our own home galaxy, the Milky Way, but contains a small fraction of the stars. Based on the mass and movement of the stars within this galaxy, scientists estimate it has little to no dark matter — in fact, there isn’t any room for it!
By observing the movement of clusters of stars within the galaxy, it appears these stars move fairly slow. Of course, slow in interstellar terms is roughly 23,000 miles per hour, but it is slower than the stars in galaxies that contain dark matter.
This galaxy also doesn’t seem to house the typical supermassive black hole that resides in the center of most spiral galaxies. Our own Milky Way has one — and possibly more than one, if recent predictions are correct. NGC 1052-DF2 doesn’t have any black holes — another thing that sets this strange galaxy apart from others in the universe.
It may have been created as an afterthought — it’s entirely possible that DF2’s creation was due to the creation of a nearby elliptical galaxy, dubbed NGC 1052. Gas traveling toward the big elliptical galaxy could have been fragmented by the black hole forming at the center of 1052, leading to DF2’s creation.
What does this discovery mean for astronomy and how we look at the universe?
It changes what we think of as galaxies and may even change our understanding of dark matter. Before this discovery, galaxies fell into two categories — spiral, like the Milky Way, and Elliptical, like NGC 1052. DF2 creates a whole new category of galaxy, one that doesn’t contain a black hole at its core, and one that appears to contain no dark matter.
Van Dokkum and his team are currently surveying 23 other ultra-diffuse galaxies similar to DF2 and at least three of them so far display the same lack of dark matter.
DF2 is roughly 65 million light years from Earth, so our studies will be restricted to enhanced imagery from the Hubble Telescope and hopefully from its sister scope, the James Webb Space Telescope which is supposed to reach orbit sometime in 2020. What it has done so far is change the way we look at the stars and may change our fundamental understanding of the universe we call home.