What is in the universe? We currently think there are between a few hundred billion and a couple of trillion galaxies in the universe. Each of these galaxies has about 100 billion stars, and all those stars have planets, and those planets have moons.
All of that, though - the atoms, molecules, and gas that make it all up - accounts for only 5 per cent of the universe. Another 25 per cent is in something called dark matter, which makes up most of the mass of galaxies. We think it is a weakly interacting massive particle. It does not emit its own light, but we can measure it through its interactions with normal matter via gravity.
