When No Evidence Is Good Evidence

Diffuse Galaxy NGC 1052-DF2 Photo credit: NASA

Diffuse Galaxy NGC 1052-DF2 Photo credit: NASA

The puzzle of dark matter is one of the great conundrums of astrophysics.

Part of the puzzle is that it cannot be seen – “dark” in astronomy means invisible. So far, it cannot be detected. It can only be inferred. When you see concentric ripples in a quiet pond, you infer someone threw a rock in recently.

It’s like that with dark matter. The movement of stars in galaxies and the movement of galaxies themselves cannot be explained unless there is a great deal more mass present than astronomers can see. Stars move within galaxies at speeds that would fling the galaxies apart unless there is immense amounts of undetected mass holding them together. So far, astrophysics has been unable to directly detect, let alone measure, what that mass might be.

That inability to detect dark mass gave rise to the idea that maybe we have gravity wrong. After all, if all galaxies are moving too quickly for the current understanding of gravity to explain, maybe the problem is the explanation. Maybe our theories of gravitation are wrong. Finding a galaxy or two where the stars don’t move faster, where you don’t need to infer dark matter to explain their motion, would disprove that idea.

There’s a diffuse galaxy in the direction of the constellation Cetus, some 62 million light years away, NGC 1052-DF2.1 Several different teams of astrophysicists have been able to gather enough light from the diffuse galaxy to measure movement. And the stars in DF2 are moving slowly, so slowly that there is apparently little or no dark matter present. Initially, scientists were skeptical of the data, but a longer baseline of data has persuaded most astrophysicists: DF2 doesn’t have much, if any, dark matter.

Now there is a second diffuse galaxy, also in the general direction of Cetus but a bit further away, which similarly doesn’t seem to have dark matter. NGC 1052-DF4, much like its cousin, seems held together only by visible matter.

All of which makes it slightly more likely that dark matter is real. If the data for inferring the presence of dark matter varies, then it’s not our theories of gravity that are broken. It’s just that dark matter is very hard to detect.

Of course, it’s more negative evidence. Direct evidence of dark matter still hasn’t been found. And the theory that dark matter acted as the attractor to create galaxies doesn’t look so strong; we now know of two galaxies – albeit diffuse, flimsy excuses for galaxies – that seem to have developed without any evidence of dark matter being present.2 We;;, perhaps it wasn’t so much a theory as a supposition.

Even more interestingly, a group from Yale has found a galaxy – DF44 – that’s about 16 million light years away, in the direction of the Coma Cluster. DF44 is as big as the Milky Way, yet it only emits about 1 percent as much light. It seems to be made of 99.99% dark matter. It’s more indirect evidence that dark matter is real, and that the amount of dark matter in a given galaxy can vary wildly.3

Dark matter, like the even more astonishing dark energy, are two of the great mysteries of science. Astrophysics continues to nibble away at them.

All of which is how and why no evidence can be good evidence.


  1. The “DF2” indicates it was discovered by the Dragonfly Telephoto Array, a telescope array system designed to detect faint objects. 
  2. As we speak, as you read this, astrophysicists are developing theories of galactic genesis that accommodate diffuse galaxies and the absence of dark matter. 
  3. WC freely admits to grossly simplifying, maybe oversimplifying, these explanations. The math is pretty fearsome, and while it is tempting to digress into discussions of red shift and blue shift to explain how motion in objects millions of light years away can be measured, that will have to be in a later post. 

3 thoughts on “When No Evidence Is Good Evidence

  1. The evidence against incompleteness in the understanding of gravity seems to be growing, but I still like MOND at an intuitive level. And note: “through rock” should be “threw a rock” in the second paragraph.

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