Over the last few decades, we’ve learned a lot of interesting things about the Universe. One of the most groundbreaking is that most of the matter in the Universe is not made up of all the stuff we know as normal matter: protons, neutrons, and electrons.
This means that atoms, the basic building block of all we know and love on Earth, make up only a small fraction of the mass in the Universe.
How do we know this? Well, there are lots of reasons, but they all boil down to these two things: we use telescopes to measure light, which tells us about the amount of normal matter in the Universe, and we also observe the motion of matter to measure gravity, which tells us about the total amount of matter in the Universe.
If these two numbers were the same, the observations would tell us so. But there’s a nail-in-the-coffin of not having dark matter, and here’s the image that proves it:
Here are two clusters of galaxies that have just collided. The pink is where X-rays are emitted, which means this is where normal atoms are colliding, heating up, and emitting light. This is what normal matter does, and we see plenty of it.
But mass also bends light, and so we can — just from looking at all the light coming from behind these clusters — figure out how much total mass there is, and where it is. And it’s in blue, which tells us two things. First off, there’s a type of matter, that’s where most of the mass is, that isn’t made up of atoms! That’s what dark matter is, and there are plenty of other colliding galaxy clusters that show this.
For instance, there’s cluster Abell 520, above, and cluster MACS J0025, below.
But there’s a second thing that we can learn from these images, which is that — unlike normal matter — dark matter doesn’t go SPLAT! when it collides!
Instead, dark matter just passes right through itself, passes right through normal matter, and only through its gravity does anything influence it at all.
So that’s what we know about dark matter, and that’s (partially) how we know it.
Now, we also know that whatever dark matter actually is, there’s a lot of it. And — what’s more — dark matter is probably its own anti-particle. Under normal circumstances, when you collide matter and anti-matter together, you get pure energy out, and an equal amount of matter and anti-matter. We see this all the time around black holes, which are anti-matter factories.
And, not surprisingly, we see this at the center of our galaxy, too. Take a look:
I say not surprisingly, because we know there’s a supermassive black hole at the galactic center! But what is surprising is that a whole bunch of scientists are blaming dark matter for this excess of energy instead of blaming the black hole!
Here’s the kicker, though. This extra energy could, in principle, be caused by either one. There’s a test we can do. If it’s caused by a black hole, we would see extra positrons, but not extra anti-protons. If it’s caused by dark matter, we would see excesses of both. Let’s go to the data. First, this experiment that’s been getting a lot of press, PAMELA, measured the positrons very accurately.
And they found them! But when they looked for extra anti-protons, here’s what they found:
Absolutely nothing! Now, PAMELA doesn’t have the sensitivity to measure this definitively, but I don’t know why everyone’s all excited, calling this dark matter, since the evidence isn’t there! If anything, this tells us that it’s more likely that there’s something interesting going on with black holes, causing them to spit out positrons!
So, yes, it’s possible that these positrons are coming from dark matter, and that evidence of anti-protons would support that. But with the evidence we have, that’s not even the most likely explanation for them! The sensationalism is painful to be surrounded by, and even though I have a special affinity for dark matter, this smells more like a black hole to me.