Dark matter makes up about a quarter of the cosmos, but we still don't know what it is. As part of a two-part series called Light & Dark on BBC Four, physicist Jim Al-Khalili pondered how close we are to understanding the mysterious "dark stuff".
Given all the progress we've made in modern physics over the past century, you may be forgiven for thinking that physicists are approaching a complete understanding of what makes up everything in our Universe.
For example, all the publicity surrounding the discovery of the Higgs boson last year seemed to be suggesting that this was one of the final pieces of the jigsaw - that all the fundamental building blocks of reality were now known.
So it might come as something of a shock to many people to hear that we still don't know what 95% of the Universe is made of. The stars in galaxies revolve around like undissolved coffee granules on the surface of you mug of coffee just after you've stopped stirring it.
It's all rather embarrassing. Everything we see: our planet and everything on it, the moon, the other planets and their moons, the Sun, all the stars in the sky that make up our Milky Way galaxy, all the other billions of galaxies beyond with their stars and clouds of interstellar gas, as well as all the dead stars and black holes that we can no longer see; it all amounts to less than 5% of the Universe.
And we don't even know if space goes on for ever, what shape the Universe is, what caused the Big Bang that created it, even whether it is just one of many embedded multiverses.