The most embarrassing mystery in science.

Matter, the stuff that makes up you, me, the Earth, the Sun, the stars and the galaxies, only makes up 4 percent of the Universe. A further 23% of the Universe is made up of dark matter and the final 73% is dark energy. Would you like to know exactly what dark matter and dark energy are? Yeah, so would scientists. That’s right, they make up 96% of the Universe and we don’t know what they are, more than a little embarrassing.

In fact the term “dark” matter comes from our inability to find it. When we look at how galaxies rotate, the speed with which the outer stars orbit the galactic centre implies the presence of more matter than we can see, especially on the edge of galaxies in a halo (like a huge sphere). So this implies there must be matter out there we can’t see – dark matter. What exactly dark matter is no body can be sure because, well, we’ve never seen any. Lots of possibilities have been suggested; vast numbers of mini black holes, starless planets or super-heavy particles but something we haven’t even thought of is just as likely.

Next dark energy. Consider a few general well known physics points:

1. The Universe expanded out of the Big Bang.
2. Gravity pulls everything together.
3. There’s lots of stuff in the Universe.

So using these three pieces of knowledge one might think that the expansion caused by the Big Bang would be opposed by the pulls of gravity between all the galaxies in the Universe. So we would expect that if the Universe is still expanding, it should at least be slowing down. Instead what we find is that the expansion is accelerating, so, there must be some unknown force or energy driving the Universe apart; i.e. dark energy.

Now it’s possible neither dark energy nor dark matter exist. It would certainly explain why we can’t work them out. But that would only be true if our ideas of gravity are wrong. Adjustments to the laws of gravity have been proposed to explain what we see (known as MOND – MOdified Newtonian Dynamics). The problem is none other these explain EVERYTHING we see and our theory of gravity (namely Einstein’s General Relativity) is incredibly successful. It is arguably the greatest achievement of the last few hundred years of science. Finding this to be wrong would be devastating and as such scientists would need a lot of convincing to abandon this theory, evidence which for now is just not there. For now then we will continue the quest to work out the as of yet unidentified 96% of Universe.