95 Percent Unknown

Look up at the night sky and you can see stars, planets, and glowing galaxies. But here's a wild fact: everything you can actually see, every star, planet, and speck of dust in the universe, adds up to only about 5% of everything that exists. The other 95% is invisible. Scientists call it dark matter and dark energy, and even though we cannot see it, we know it's there because of the effect it has on everything around it.

- What dark matter is and how scientists know it exists without seeing it - What dark energy does and why it's different from dark matter - How galaxies gave scientists their first big clue - Why the word 'dark' really means 'invisible,' not spooky

In the 1970s, astronomer Vera Rubin studied how fast stars orbit around the edges of spiral galaxies. Based on the visible matter alone, stars far from the center should orbit much slower than stars near the center, the same way outer planets in our solar system orbit the Sun more slowly than inner ones. But Rubin found something strange: stars at the edges of galaxies were moving just as fast as stars near the center. The only way to explain that is if there's a huge amount of extra mass in each galaxy that we cannot see, extra gravity pulling on those outer stars and speeding them up. Scientists named this invisible material dark matter.

Dark matter does not give off light, so telescopes cannot see it directly. But its gravity bends light from distant galaxies behind it, an effect called gravitational lensing, and scientists have photographed this bending many times, giving strong evidence dark matter is really there. Scientists estimate dark matter makes up about 27% of the universe, roughly five times more than all the regular matter (stars, planets, gas, and dust) combined, which is only about 5%.

In 1998, two teams of astronomers studying exploding stars called supernovae discovered something even stranger: the universe isn't just expanding, its expansion is speeding up. Something is pushing galaxies apart faster and faster over time. Scientists call this mystery force dark energy, and it makes up roughly 68% of the universe, the largest slice of all. Nobody fully knows what dark energy actually is yet, it remains one of the biggest open questions in all of science.