Dark matters

There's a lot of weird stuff called dark matter in the universe. Its gravitational pull is what holds our galaxy together, along with all the other galaxies we've investigated.

Some folks accuse scientists of unnecessarily complicating things, and all but making up weird theories about dark matter and dark energy.

Well, not only did the not "just make this stuff up", scientists are starting to uncover some of the properties of this stuff.

Dark matter has evaded all attempts to detect and illuminate it in the 73 years since its existence was first hypothesized. Now British astronomers have moved an important step closer to lifting the veil on the elusive material that has mystified generations of scientists, by calculating some of its basic physical properties for the first time.

By studying the movement and mass of stars within these galaxies, they were able to calculate the minimum density and distribution of the dark matter around them. <snip> astronomers calculated that they moved at about six miles per second, giving it a "temperature" higher than the surface of the Sun. If it was made of hydrogen atoms, dark matter would be as hot as 10,000 C. Thanks to its unusual nature, it has a high temperature caused by the excited movement of its particles but no heat. Prof Gilmore added: "The strange thing about dark matter is that it has temperature but doesn't give off radiation. It is a different form of matter not made of the same stuff as ordinary matter that consists of protons and neutrons, and has no charge."

So how do we know all this?

Part of it has to do with the assumptions physicists make when they try to explain the universe.

One of the fundamental assumptions is that the universe is pretty much the same everywhere – it obeys the same rules, matter has the same properties, forces behave the same way, and so on.

And that seems to be true, as far as we can see. Stars emit light in the same way our sun does, and material emits and absorbs light in precise spectra the same way it does in the lab. Mass, force, and motion seem to work together in distant clusters of stars the same way it does in the lab.

And so on.

We know there's dark (invisible) stuff clustering in and around galaxies. We can measure how fast galaxies are spinning. If we add up the mass of the stuff we can see in galaxies, we find there's far too much mass to keep the stars from being flung off into intergalactic space by centrifugal force. Since we don't see galaxies flying apart, and we don't see fragments of splintered galaxies, something is providing enough force to hold them together.

The simplest explanation is that galaxies are held together by gravity, and the only (known) way to produce a lot of gravitational force is with a clump of something that has mass. So there's matter scattered around these galaxies, and its gravitational pull holds the galaxies together. And we can't see it. We can only infer its presence from the direction in which it tends to pull stuff we can see.

Since this stuff doesn't seem to emit light, or radio waves, or x-rays, or any other kind of electromagnetic radiation, no matter what happens to it or what kind of hot object it's next to, we conclude that it has no electric charge at all. And this is more than just being elecectrically neutral, like a hydrogen atom. Hydrogen atoms, and in fact most matter, is electrically neutral because its positive and negative charges are in balance. Electrons and protons are found in equal numbers – or darn close to it – in all matter. Besides electrons and protons, there are neutrons which have no net electric charge. However, that's "net" electric charge because neutrons are composed of particles which do have a charge. (The first clue was that neutrons are affected by magnetic fields.)

We know dark matter has no charge because when you accelerate charged particles, they emit light. No matter what you do to dark matter, it won't emit light.

Conclusion: either dark matter is weird, and behaves like nothing else we're familiar with, or the rules are profoundly different in parts of the universe not that far away from us.

Given a choice between "making up" something like dark matter or assuming that laws of nature are only "mostly" universal, scientists prefer the former.

It can lead to pretty bizarre results, but that's how science works.