In the dark about Dark Matter

In Einstein’s theory of general relativity all matter (mass, heavy things, like you and I) generates a gravitational field around them; i.e. they curve, bend (and sometimes twist) space and time around them. How space and time is shaped by the mass (and energy, recall that E=MC^2, so matter and energy are very much linked) determines how everything from falling apples to planets and galaxies move about.

Now, if we look around us and sum up all the matter we can see in galaxies, stars, gas clouds etc. we can use mathematical models (Einstein’s equations) to model how things should move and then compare it to what we actually observe.
We could, for example, observe a galaxy through one of the many powerful telescopes we now have at our disposal, measure the amount of mass in it and check if it moves in a way consistent with all that mass.

Scientists have done just that. And they have found, to their great surprise, that many of these galaxies don’t quite move the way they are expected to.
Instead they move (rotate, to be specific) in a way which seem to indicate that there’s more mass in them than we can see…

So what’s going on? Are Einstein’s equations, or indeed most of our assumptions about “how stuff works”, wrong?

Well, Einstein’s equations and our assumptions could certainly be wrong but so far they have withstood everything we’ve thrown at them and results predicted by theory (i.e. worked out on paper) have matched actual observations with mind blowing accuracy.

So let’s not assume they’re all wrong, that would almost be “too easy”.

That really only leaves one other possibility; that there is mass out there in the galaxies, or in the universe as a whole, that we just can’t see…

Enter Dark Matter.

To clarify; “Visible” matter is everything that interacts with stuff around it and which generates effects that we can ultimately observe as some sort of light. A shining star is a pretty obvious example and another are moons and interstellar gas clouds that absorb or reflect light and radiation and therefore give away their presence.

We ourselves are made up of visible matter.

Invisible matter on the other hand would be something that has mass, in other words has an effect on gravity, but that does not react with anything visible. Think about it this way; these things (whatever they are) could pass through something as big as the sun without as much as slowing down or causing even a single photon in the star’s interior to blink. They’re just there and the rest of us (who belong to the “visible matter” part of the Universe) are none the wiser.

That is, until we take a closer look and detect their gravitational effects such as their effect on galaxies that don’t quite move the way we would expect them to.

There are a number of theories about Dark Matter, of course, each with their strengths and weaknesses but fundamentally they all center around the idea that the structure of the universe we see today condensed out in some way as the universe cooled down after the Big Bang. And,  once again, the theories require there to be more matter present in the universe than what we can see.

Physicists don’t really know what Dark Matter is or even how to detect it directly; after all it does not  seem to react with anything other than through gravity. There are also many different candidates for what Dark Matter could be made of, depending on which theory you believe in; there is something called “Hot Dark Matter” for which a very light elementary particle called the Neutrino might be a candidate. Then there is “Cold Dark Matter” which could be anything from exotic – yet unknown – elementary particles called “WIMP”s (Weakly Interacting Massive Particles) to masses of very faint objects, like small planets and black holes.

As always the only way to settle on a theory is to test its predictions and then test them again…and again. Which of course, is exactly what physicists and cosmologists are doing.

Finding Dark Matter, conclusively, would be a tremendous achievement. It would mean that we would have tested the accuracy of some of our theories, like Einstein’s and Quantum Mechanics, in a truly monumental domain; that of the large scale behaviour of our own universe.
Not finding it, on the other hand, or proving the theory wrong, would be just as monumental although it would, of course, mean that we would have to go back to the drawing board…

Some of the latest experiments (at the time of writing) claim to have found traces of Cold Dark Matter…these are exciting times for science indeed!

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