Five great unsolved mysteries of the universe

Where are all the aliens?
Here’s the deal: There are about a hundred billion galaxies in the universe, each containing tens of billions of stars. Many have planets, so even if life only exists on one planet out of every trillion, that still leaves the number of life-bearing planets in the order of one billion. Why can’t we find them? Few planets in our Solar System seem promising for microbial life—maybe Mars or the subsurface ocean of Europa. But does extraterrestrial intelligence require biology? Discuss.

How did the universe happen?
Whether or not you believe in the Big Bang—the theory that in a fraction of a second, the Universe exploded into being from smaller than a single atom—it still doesn’t explain what came before it and caused it. Matter doesn’t just explode from nothing—or does it? Scientists have some idea of the universe when it was 300,000 years young, but we still have a long way to go, especially because we don’t fully understand the quantum aspects of gravity.

What’s up with dark matter?
We know it exists because we can see how it affects gravity on the visible stars and galaxies, but what is dark matter? It’s invisible, but it’s believed to make up about 27 percent of the universe. How can that be? How can we see the rest of the iceberg?

Or dark energy?
So what about the rest of the matter in the universe? Cosmologists think its “dark energy.” If we think of dark matter as what pulls everything together, dark energy is what pulls it all apart. But nobody knows what it’s made of–even though we know it affects the universe’s expansion.

Why do we sleep?
Wait, what? Scientists can’t offer us a definitive answer? Many theories abound: Some believe that sleep helps organize the brain to aid memory or that it helps the brain flush out toxins or that it helps us restore and repair the body. Even though we don’t know the actual reason, we do know that we need it to survive.