Galaxies and dark matter go together like peanut butter and jelly. You typically don't find one without the other.
Therefore, researchers were surprised when they uncovered a galaxy that is missing most, if not all, of its dark matter. An invisible substance, dark matter is the underlying scaffolding upon which galaxies are built. It's the glue that holds the visible matter in galaxies — stars and gas — together. ...
The unique galaxy, called NGC 1052-DF2, contains at most 1/400th the amount of dark matter that astronomers had expected. The galaxy is as large as our Milky Way, but it had escaped attention because it contains only 1/200th the number of stars. Given the object's large size and faint appearance, astronomers classify NGC 1052-DF2 as an ultra-diffuse galaxy. A 2015 survey of the Coma galaxy cluster showed these large, faint objects to be surprisingly common.
But none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is an oddball. ...
After drawing both praise and skepticism, the team of astronomers who discovered NGC 1052-DF2 – the very first known galaxy to contain little to no dark matter – are back with stronger evidence about its bizarre nature.
Dark matter is a mysterious, invisible substance that typically dominates the makeup of galaxies; finding an object that’s missing dark matter is unprecedented, and came as a complete surprise.
“If there’s one object, you always have a little voice in the back of your mind saying, ‘but what if you’re wrong?’ Even though we did all the checks we could think of, we were worried that nature had thrown us for a loop and had conspired to make something look really special whereas it was really something more mundane,” said team leader Pieter van Dokkum, Sol Goldman Family Professor of Astronomy at Yale University.
Now, van Dokkum’s team has not one, but two, new studies supporting their initial observations, demonstrating that dark matter is in fact separable from galaxies. ...
Ironically, the lack of dark matter in these UDGs strengthens the dark matter theory. It proves that dark matter is a substance that is not coupled to ‘normal’ matter, as both can be found separately. The discovery of these galaxies is difficult to explain in theories that change the laws of gravity on large scales as an alternative to the dark matter hypothesis. ...
Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC 1052-DF2 ~ Shany Danieli et al
Like DF2, DF4 belongs to a relatively new class of galaxies called ultra-diffuse galaxies (UDGs). They are as large as the Milky Way but have between 100 to 1000 times fewer stars, making them appear fluffy and translucent, therefore difficult to observe.
Ironically, the lack of dark matter in these UDGs strengthens the dark matter theory. It proves that dark matter is a substance that is not coupled to ‘normal’ matter, as both can be found separately. The discovery of these galaxies is difficult to explain in theories that change the laws of gravity on large scales as an alternative to the dark matter hypothesis.
These two Ultra-diffuse Galaxies share at least four things in common; large size, low star density, little or no dark matter, and location in the same cluster.
They help prove dark matter is real, and they show that our galaxy (and the universe) would be a much duller place without DM.