Can Galaxies Form Without Dark Matter?

For decades, theoretical astrophysicists have been telling us that dark matter is fundamental to everything we know about the universe —- that this exotic unseen matter is even needed to explain our own Milky Way Galaxy’s structure, dynamics and rotation.

In other words, the observed motions of these galaxies and clusters of galaxies’ exceed what can be explained the galaxies’ visible mass.

Yet in the last few years there are signs that this theoretical dark matter dog will no longer hunt. Even though an estimated 85 percent of matter in the universe is thought to be in the form of exotic dark matter (or non-visual matter), it has still yet to be directly detected. And observational astronomers are now reporting that indeed some dwarf galaxies, at least, appear not to need this non-baryonic dark matter to form and to function.

What gives?

For perspective, I turned to Stacy McGaugh, an astrophysicist at Case Western Reserve University in Cleveland, Ohio. Mcgaugh, a longtime dark matter researcher, has for decades been somewhat of a contrarian when it comes to accepting conventional explanations for this exotic matter.

The galaxies that appear to have little or perhaps no dark matter are not grand design spirals, says McGaugh. They are tiny dwarfs that may have been stripped of dark matter in some kind of tidal interaction (galaxy on galaxy violence), he says.

But can a full-size galaxy like a grand design spiral hold together without non-baryonic dark matter?

No, you need something extra —- beyond the mass we see —- to hold galaxies together amidst their observed internal motions, says McGaugh. The something extra could be invisible mass (dark matter) or stronger effective gravity, he says.

But unfortunately, to date, exotic dark matter can only be detected by its gravitational effects. Dark matter makes the outer regions of grand spiral galaxies like our own milky way rotate faster than expected given their measured total starlight. And it can make clusters of galaxies orbit faster than expected given their total starlight.

Yet could there be enough normal unseen matter —- in the form of neutron stars, white dwarfs or even very faint red dwarf stars, for instance; that could explain spiral galaxy formation without exotic dark matter?

No, we need new physics, says McGaugh. Again, it could be exotic dark matter or a modification of gravity, he says.

To be sure, some normal matter is probably hidden in unseen forms, but it almost certainly can’t be enough to solve our problems, says McGaugh. For one, we have a pretty good idea of how much normal matter there is from big bang nucleosynthesis, he says. For another, we can usually figure out a way of detecting normal matter, even if it is well hidden, notes McGaugh.

The microlensing observations that limit the number of dark stars intervening between us and the visible stars in the nearby Magellanic Clouds are one example, says McGaugh. If there were lots of white dwarfs or neutron stars or even black holes composing unseen, normal matter in the milky way, we’d have detected the effects of their gravity on the light from more distant stars.

But the search for exotic dark matter continues, even though it appears to be fruitless.

“Exotic dark matter has not been found,” said McGaugh. “The experiments have already been done and have succeeded in failing.”

Even so, such experiments have achieved much greater sensitivity that they were predicted to need in order to detect Weakly Interacting Massive Particles (WIMPS). That is, subatomic particles hypothesized to make up exotic dark matter as originally conceived.

“The original WIMP hypothesis has already failed,” said McGaugh.

Will researchers give up or keep looking for these exotic dark matter particles?

A lot of people have kept looking anyway, says McGaugh. They simply say, “we don’t know what the dark matter is, but we’re sure it’s there,” he says. This is wrong on many levels, says McGaugh. First, we’re not sure it is there – we’re just sure there is a problem – either there is dark matter, or we don’t fully understand gravity (or more generally, dynamics), he says.

It is the theoretical community that keeps moving the goalposts, says McGaugh. It has become a self-perpetuating industry: immune from external criticism, unconcerned with the astronomical data that launched the whole endeavor, and entirely too big to fail, he says.

And if dark matter doesn’t exist, how do grand spiral galaxies like our own Milky Way ultimately form?

“You need new physics,” said McGaugh. “Stronger effective gravity could do the trick. It might even make galaxies form faster.”

That could mean that fully formed large galaxies would appear at even higher redshifts than previously thought. If so, NASA’s James Webb space telescope should be able to see this directly, says McGaugh.