We just Discovered A Galaxy Without Dark Matter, Proving it Exists

Dark Matter has been a persistent thorn in the side of physicists. It shouldn’t be there, but it has to be, and obviously is. And, even weirder, dark matter is far more common than ordinary matter. We can trace it thanks to a property of space-time called gravitational lensing, and as far as we can tell, it’s everywhere. Everywhere, that is, except one galaxy: NGC1051-DF2.

The celestial body sits about 65 million light years away and is about the same size as our own Milky Way, but with less than half a percent as many stars as we do.  That;s already quite strange as it means the galaxy is super low-mass. But, scientists also don’t see any dark matter around it. All other observed galaxies have a halo, or other formation of dark matter around or through them.

“Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy,” lead author Pieter van Dokkum of Yale University told Science Alert.

This computer-simulated image shows a supermassive black hole at the core of a galaxy (via NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI))

“For decades, we thought that galaxies start their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC1052-DF2 challenges the standard ideas of how we think galaxies form.”

That’s important because until now, dark matter has been so elusive that some scientists thought that it might not exist, and that our equations about gravity (i.e., Einstein’s equations) were wrong. But they aren’t. Gravity behaves in the same weird way we’ve thought for almost 100 years, but now we have to figure out how you have a dark matter-less galaxy.

“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,” van Dokkum said. “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.”

The only decent hypothesis on the table right now is that a larger, nearby galaxy called NGC 1052 was pulling gas inwards, and that the smaller NGC1052-DF2 was simply the result of a fragmented cloud of gas. But even that doesn’t tell the whole story, and doesn’t account for all the bizarre properties DF2 has.

Dark matter

“Every galaxy we knew about before has dark matter, and they all fall in familiar categories like spiral or elliptical galaxies,” van Dokkum said. “But what would you get if there were no dark matter at all? Maybe this is what you would get.”


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