The universe weighs less than we thought

When the world's best scientists decided to team up and measure the mass of the universe all the way back in the 1970s, they set themselves a pretty tall challenge. Applying their best understanding of gravity and the dynamics of galaxies, though, they came up with an answer — an answer which sadly predicts our universe should be falling apart. We know that 

galaxies' matter orbits a single central point — we've observed it! — and that must mean their own motion generates enough centripetal force to make that happen.

But calculations suggest that there's not actually enough mass in the galaxies to produce the forces required to keep themselves moving in the way we've observed. So physicists scratched their heads, worried a little, then proudly stated that there must be more stuff out there than we can see. That's the theory behind what everyone now refers to as Dark Matter. The only problem? In the past 40 years, nobody has confirmed whether it really exists or not—so, effectively, the problem thrown up by those initial calculations remains.