Dark matter is estimated to make up about 85% of all matter in the universe, so scientists were surprised when, in 2018, they found a galaxy where dark matter seemingly only makes up a few percent of the matter content. According to a press release in 2018, the galaxy, named NGC 1052-DF2 but commonly referred to as just "DF2." contains at most 1/400th the amount of dark matter it was expected to contain.
The team reached the conclusion based on the motions of the stars within the galaxy as their velocities are influenced by the pull of gravity. The scientists found that the observed number of stars accounts for the galaxy's total mass, leaving not much room for dark matter.
"For almost every galaxy we look at, we say that we can't see most of the mass because it's dark matter," said Pieter van Dokkum, the leader of the team that first reported on the odd galaxy, according to NASA. "What you see is only the tip of the iceberg with Hubble. But in this case, what you see is what you get. Hubble really shows the entire thing. That's it. It’s not just the tip of the iceberg, it's the whole iceberg."
Scientists at first cast doubt on the findings, as they could be explained by an inaccurate measurement of the galaxy's distance from the Earth. If the galaxy were closer to Earth than initially thought, it would be intrinsically fainter and less massive and would need dark matter to account for the observed effects.
But new findings from the Hubble telescope have helped confirmed that not only is DF2 as far as astronomers initially thought, it's actually further than the original estimates. The galaxy is now estimated to be about 72 million light-years from Earth, seven million light-years further than initially thought.
The galaxy is also odd because it doesn't seem to have a noticeable central region, spiral arms or a disk which spiral galaxies usually have, but doesn't look like an elliptical galaxy either. The galaxy also doesn't seem to have a central black hole. The clusters of stars in the galaxy are also odd, as they are about twice as large as typical stellar groupings in other galaxies.
"It's like you take a galaxy and you only have the stellar halo and globular clusters, and it somehow forgot to make everything else," said van Dokkum. "There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown."
Another galaxy, NGC 1052-DF4 (also known as just DF4), also seems to be lacking dark matter, although this may be due to the dark matter having been stripped out of the galaxy due to tidal forces from another galaxy.
Scientists believe that both galaxies were members of a collection of galaxies, but new observations by Hubble show that the two are 6.5 million light-years apart, further than first though.
"Both of them probably were in the same group and formed at the same time," said Shany Danieli of the Institute for Advanced Study in Princeton, New Jersey, according to NASA. "So maybe there was something special in the environment where they were formed."
The search is on for more galaxies lacking dark matter, with more work needed in order to understand how these galaxies formed without dark matter.