Our Universe isn’t like us. While we’re made of atoms, and other forms of normal matter, cosmic-scale observations indicate that the overwhelming majority of the Universe is dark. There’s dark energy, driving the expansion of the Universe, and dark matter, holding the massive clumps and clusters together. Every known galaxy contains about five times as much dark matter than normal matter, leading to the gravitational phenomena we observe.
But there are exceptions. Earlier this year, astronomers revealed one of the most puzzling galaxies ever found, NGC 1052-DF2. It’s diffuse, as large as the Milky Way, but with less than 1% of our stars. Early observations indicated that it was missing most — if not all — of its dark matter. But does this galaxy really present a challenge for dark matter, or is something more complex going on? As 2018 comes to a close, here’s what we’ve learned so far.