The Mystery of Dark Matter

NASA galaxy image

The missing mass problem

by Spike Psarris

What is dark matter?

Many astronomers will tell you that it’s a mysterious substance, as yet unknown to physics. It has many strange properties, including invisibility. And the Universe contains a lot of it: there’s four to five times as much dark matter as there is ordinary matter (the stuff that makes up stars, planets, and so on). In other words, for every galaxy you see, there’s four or five galaxy’s worth of stuff that you don’t see.

But other scientists disagree with this.

Some will tell you that dark matter doesn’t really exist. Instead, the observations that are claimed to show the existence of dark matter are better explained in other ways.

What’s going on here?

I discussed dark matter briefly in Volume II and again in Volume III. Although I mentioned that there were alternate explanations, I didn’t have time then to discuss them. So let’s do that now.

Why Many Astronomers Believe in Dark Matter

There are places in the cosmos where we observe things moving more quickly than perhaps they should.

For example, a typical galaxy contains about 100 billion stars. These stars are all orbiting their common center of mass—basically, the center of the galaxy. We can use celestial mechanics (the physics of motion for astronomical objects) to calculate the velocities of stars within the galaxies.

But when we measure the stars’ actual speeds, they don’t match this graph. The inner stars are moving as we expect, but the outermost ones are moving more quickly.

By the way, this is called a galaxy rotation curve: a graph of the orbital speeds of stars, versus their distance from the galaxy’s center. And it turns out that ‘discrepant’ curves are common.

Many galaxies have this divergence between the outer stars’ expected speeds and their observed speeds.

So what’s going on here? One possible explanation is that there is a lot of matter—a lot of physical stuff—arranged in a “halo” (a spherical shape surrounding these galaxies).

If there were enough matter in a halo, then its mass would gravitationally affect the galaxy’s stars, and this would alter their orbits into the patterns that we observe.

However, this would require a lot of mass—several times the amount of mass that the galaxy itself contains. And even after exhaustive searching for halos around our galaxy and others, we don’t see anything remotely close to the amount of mass required.

So, it appears that there is a lot of mass that’s missing. In fact, this used to be called the missing mass problem.

Today, it’s usually not called that anymore, because most astronomers say that the mass isn’t really missing. They say that there is indeed a lot of matter surrounding these galaxies.

We’re just unable to see it.

This has become known as dark matter. It’s physical stuff, thus the word “matter”—but we don’t see it, thus the word “dark.” …

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image credit: NASA galaxy image