Some 65 million light-years from Earth lies a galaxy-sized mystery — one among many — that astronomers are working to solve. The galaxy, named NGC 1052-DF2, is about the same size as our own Milky Way, but it only has about 1/200th as many stars, said NASA.

That, however, is not the real mystery. Other large, very faint galaxies have been discovered, known as ultra-low surface brightness galaxies, even though their lack of many stars makes them hard to detect.

But NGC 1052-DF2 is also all but devoid of so-called dark matter. And since dark matter in space is believed to be a fundamental building block of all galaxies, the mystery lies in how this particular galaxy can exist at all.

A Massive Mystery

Dark matter is very peculiar stuff and one of our most remarkable space discoveries. We cannot see it or directly detect it with any instrument we have. It does not glow or reflect light from nearby stars or even block the light of stars behind it, according to NASA.

So “dark matter” does not refer to how it appears in a telescope. Instead, as astrophysicist Alberto Conti of Northrop Grumman Aerospace Systems explained, “In astronomy, if we don’t know what something is, we call it ‘dark.'”

If we don’t know what it is, and can’t even see it, how do we know it is there at all? The answer is gravity. We can measure the motion of stars and clusters in distant galaxies to determine their orbital speed, said NASA.

These calculations show that all known galaxies — except for NGC 1052-DF2 — are much more massive than their visible stars, gas and dust and other objects combined. In fact, most of the mass in the universe is in the form of dark matter, detectable only by its gravitational influence, NASA noted.

The Glue That Holds the Universe Together

Without all that otherwise undetectable dark matter, most galaxies would have long ago fallen apart as their stars hurtled off at high speeds into the intergalactic void. Worse than that, without the mass of dark matter to hold things together, galaxies and stars should never have formed in the first place, Conti explained.

All of which makes NGC 1052-DF2 a faint but enormous mystery. According to Science Daily, it has approximately 1/400th of the expected amount of dark matter. Dark matter is mysterious enough to begin with, but its absence is even more mysterious.

Some astrophysicists, according to Conti, have proposed alternatives to dark matter in order to explain how galaxies hold together — and how they formed in the first place. One proposal, called modified gravity, suggests that gravity is stronger on enormous scales — such as galaxies — than on the ordinary scale of stars and planets.

Surprisingly, the missing dark matter of NGC 1052-DF2 actually strengthens the argument for the presence of dark matter in space, Conti said. If Newton’s laws of gravity work differently on the scale of galaxies, he explained, then “modified gravity” should also apply to NGC 1052-DF2, since it is as big as the Milky Way. But the slow orbital speeds of its stars are consistent with the standard theory of gravity. So, what happened to its dark matter?

The Search for Subtle Oddities

To find out, astronomers are taking a closer look at NGC 1052-DF2, as well as looking at other exceptionally large and faint galaxies to see if they also show the telltale slow speeds that indicate missing dark matter. Is this galaxy a mere fluke, asked Conti, or the first example of a pattern?

One oddity that researchers have noticed, said Sky & Telescope, is that NGC 1052-DF2 has unusually bright globular clusters. These are large groupings of up to hundreds of thousands of stars that formed together. Globular clusters are common in galaxies, and have a typical range of size and brightness. Unusually bright clusters may, suggested Conti, be a marker that this galaxy is younger than most — and perhaps formed under conditions that stripped away most of its dark matter.

Classic mystery stories — including space discoveries — usually end with an explanation of how the clues come together to solve the mystery. But in the case of NGC 1052-DF2 and the missing dark matter in space, the detectives are still working on the case.