Let’s pull together what we know about gravity and the Universe. We know that gravity likes to pull things together – attract them to one another – and we know (or, at least, we think) that the Universe was formed in a Big Bang. So after the Big Bang, all the matter in the Universe has been moving further apart, away from the explosion’s centre (therefore the Universe is actually getting bigger) – but each particle of matter is also attracted to all the other bits of matter, as well.
So, logically, gravity should be slowing down the speed at which everything’s moving further apart – slowing down the expansion of the Universe. But – and here’s the problem – this isn’t what’s happening. The Universe’s expansion is actually accelerating, so day-by-day stuff is actually moving further apart at a faster speed than the day before. And the really interesting part is that no-one knows why.
This is where dark energy comes in. The reason it’s called ‘dark’ is because no experiment has been able to find evidence of it, therefore we have yet to ‘see’ it and find out what’s causing it. Scientists can calculate how much energy is needed to make the Universe’s expansion accelerate at its current rate – so we know how much dark energy there is – but there’s still no actual experimental evidence. The reason “dark energy” sounds so mysterious is because that’s exactly what it is – a mystery, even if it’s a mystery we can define some properties of.
You might think that this isn’t a big deal, and indeed it might be less of one, if it wasn’t for the fact that around 68% of the Universe is made up of dark energy (bear in mind that matter is equivalent to energy, so we can say this).
There are various theories of what the dark energy could represent: it could be that space itself, the vacuum we always assume to be empty, has some form of energy that we’ve never detected before: and one of Einstein’s theories of gravity suggests that more space can be created, thus creating more energy. Another theory suggests that there is a form of energy we simply don’t have the technology and understanding to detect, something that fills all of space but isn’t the same as normal energy; and finally, there have been suggestions that Einstein’s theory of gravity isn’t correct, and we might have to look for a new way of understanding gravity and its properties.
In the same way that there’s an unexplained type of energy in the Universe, there’s also an unexplained type of matter: dark matter. This is something we can’t detect, but scientists know it’s there: it has to be there for galaxies to form and move in the way they do (at least with our current knowledge of gravity). This mysterious matter takes up around 27% of the Universe. There are a few more options for what this could be: new particles we haven’t found yet, or incredibly dense clumps of particles that we do know exist, but not in that form. But we know for sure that it’s not anything we’ve seen before.
Those of you who are more mathematically-inclined will have already worked this out: 68% dark energy + 27% dark matter is 95% of the Universe, leaving galaxies, stars, planets and us taking up a tiny 5% of the total amount of stuff in existence. That’s why dark matter and dark energy are so important: they are a huge part of understanding the Universe we live in.