Topics: Astronomy, Astrophysics, Cosmology, Dark Matter, Theoretical Physics
Note: I almost didn't blog about this, because the original links at Live Science and Cosmos Magazine lead to "page not found" errors. I was able to find the article on Nature's direct website and provide it here. It's strange both sites had the same bogus links.
Here's a problem: The universe acts like it's a lot more massive than it looks.
Take galaxies, those giant, spinning masses of stars. The laws of motion and gravity tell us how fast these objects should turn given their bulk. But observations through telescopes show them spinning way faster than we'd expect, as if they were actually much more massive than the stars we can see indicate.
Astrophysicists have come up with two main solutions to this problem. Either there's a lot of mass out there in the universe that we can't detect directly, mass scientists call dark matter, or there's no dark matter out there, but there is something missing from our laws of gravity and motion. Researchers call the second proposed solution modified Newtonian dynamics (MOND), which suggests that if the laws are properly tweaked, the universe would make sense without dark matter.
A new paper, published today (March 28) in the journal Nature, provides compelling evidence that there really is dark matter out there and that modifying the laws of physics wouldn't by itself solve the universe's weight problem.
In that study, the researchers found an object that could exist in a universe that has dark matter, but that would be nearly unimaginable in a MOND universe: a totally normal galaxy, one that seems to operate without any dark matter-type forces. 
In a study published in the journal Nature, scientists have found a galaxy that appears to contain no dark matter — the unknown material thought to be common in the universe because of its gravitational effect on normal matter.
It was a startling discovery, because galaxies similar to our own Milky Way generally appear to contain 30 times more of the mysterious substance than normal matter, while smaller galaxies can contain up to 400 times as much.
The dark-matter-free galaxy, called NGC 1052-DF2, lies 65 million light years away in the constellation Cetus. It initially caught the attention of astronomers because, while it’s about the size of the Milky Way, it contains only 0.5% as many stars.
“That makes it very diffuse,” says the study’s lead author, Pieter van Dokkum of Yale University, in Connecticut, US. “You can look straight through it. You can see galaxies behind it.”
It was discovered by a special, low-tech telescope in New Mexico called the Dragonfly Telephoto Array, which consists of a bundle of 400-millimetre camera lenses of the same type used by sports photographers, and can scan the sky for large, dim objects. So far, it’s found 23 of them, but NGC 1052-DF2 (the DF is for “Dragonfly”) stood out because it wasn’t just a big, diffuse blob. 
1. Astrophysicists Claim They Found a 'Galaxy Without Dark Matter', Rafi Letzter, Live Science
2. Found: a galaxy devoid of dark matter, Richard A Lovett, Cosmos Magazine