Our first mystery arose in 1933, when Swiss-born astrophysicist Fritz Zwicky attempted to estimate the total mass of a cluster of galaxies. But he encountered a problem. As background, the more mass an object has, the greater it’s gravitational pull – the force that attracts other objects. The greater an object’s gravitational pull, the faster objects rotating around it will move. Zwicky noticed that galaxies at the edge of the cluster were rotating way too fast for the mass he calculated; to account for speed, the cluster had to be about 400 times more massive.
To explain this mystery, Zwicky – and many physicists after him – inferred that there’s something taking up space in the universe other than matter we can see: dark matter. Dark matter doesn't interact with electromagnetic forces, so it doesn’t reflect any kind of light or radiation; it can’t be directly observed in any way. However, some researchers think they’ve “seen” dark matter through a phenomenon called gravitational lensing: when a far away object’s mass – including dark matter – visibly bends the light it gives off.
We owe our second mystery to supernovae – incredibly bright, exploding stars. Physicists know the actual brightness of every type 1a supernovae, because the explosions always occur in stars of the same mass. Based on this actual brightness, one can compare how bright a supernova appears from earth to calculate how far away it is. Here enters the problem: these supernovae were a lot further away than physicists assumed they would be.
It’s long been accepted that the universe is constantly expanding; these supernovae's increased distance suggests it’s not only expanding, but that expansion is accelerating. To explain this, physicists propose a hypothetical dark energy that pervades throughout the universe, accelerating its expansion.
Dark energy and dark matter are astonishing as concepts; what’s even more fascinating is how much of this stuff physicist expect exist. Of the universe’s total energy-mass density (all existing matter and energy), scientists predict that 72.8% is dark energy, 22.7% is dark matter. That leaves only 4.6% for stars, planets, and every other type of matter we’ve ever seen or interacted with. Whoa.
These concepts are still considered hypothetical and inferred, but are widely accepted. Accepting answers you can’t actually see might seem like a pretty big leap of faith. Truthfully, answers to big questions and never easy to see – but its human nature to search for them any way. So the next time you find yourself riddling over one of life’s great mysteries, why not take a moment to appreciate that you don’t know the answer. After all, it’s the not knowing that makes learning so worthwhile.