Two questions that do come up quite frequently are those about Dark Matter and Dark Energy, what they are and what we know about them. So today I would like to take the time to delve deeper into the mysterious darkness of the universe! As this is a fairly extensive question I think it would be prudent to split it into two parts.. Here we go!
Part 1: Dark Matter
So what is this dark matter stuff anyway? The name does go and give a fairly apt explanation of what it is, believe it or not. To prove this, I am going to get out my trusty dictionary.
- with little or no light : it's too dark to see much.
- hidden from knowledge; mysterious.
- physical substance in general, as distinct from mind and spirit; (in physics) that which occupies space and possesses rest mass, esp. as distinct from energy.
So Dark Matter is nothing more than a non-light emitting substance with a rest mass energy. And just to clarify, when we use the term "light" what we are actually referring to is a photon. This means that dark matter doesn't emit anything detectable from the lowest part of the spectrum (radio waves) to the highest part (gamma rays). So the first question you might ask is... If we can't see it how do we know it exists? To answer this we need to go and look at the rotational curves of the Milky Way (the galaxy in which we reside) as seen here:
Curve A: This what we would expect to see if it wasn't for dark matter. The further out you get from the center of the Milky Way the slower things start to move. This principle not only holds true for the way in which a star rotates around a galaxy but also the way a planet rotates around a star. The same mathematical principle governs the two, so we would expect the both to be the same (curve A).
Curve B: This is what we actually see when we look at the speeds of stars within our Milky Way and we don't just see it here, we see the same phenomena in all galaxies! What we can deduce from this graph is that there must be more mass than we are able to see. Here enters dark matter.
So far the only method that we've actually been able to detect dark matter is through it's influence on physical matter through gravity. In this way it's sort of like the wind-- looking out of a window we can see the leaves and trees moving, but we can't see the air blowing against it. The detection of high energy particles also hit to the potential detection of dark matter. High energy x-ray's have detected something at the energies 130 GeV (Max Plank Institute for Physics), 620 GeV and 820 GeV (Calorimetric Electron Telescope). To give you a bit of an idea as to the mass of 620 GeV, it equates to about 37 times heavier than water.
Although the 130 GeV that was detected was put as a potential for dark matter, even more recently it has changed hands and been dubbed the Higgs Boson! (Higgs Boson is more accurately put at 125 GeV) That is the brilliant thing with science, always so much more to learn! There is so much more that could be said about dark matter, but to conclude: it is currently virtually undetectable, but we can see it's affects because of its gravitational influence. It is everywhere in the universe because it affects not only our galaxy but every other galaxy (detectable because of gravity). We don't quite know what it is but we have several potential candidates.
Check out part 2 here: