10 Biggest Unexplained Mysteries Of The Universe

7. Planet Nine

Planet Nine is not so much a mystery, but a theory to explain a mystery.

In our solar system, there are a number of things that just don't add up, which, as we've discovered so far, is apparently not that uncommon.

The orbits of objects really far out in the solar system are noticeably skewed to one side, leaving a massive gap on the other side of the sun. This could only really be the case if a larger object was, in effect, pushing everything to the opposite side, which is the main argument for the existence of a ninth major planet.

Add to this a sprinkling of regular, periodic extinction events on Earth and a dash of gravitational irregularities, you've got a recipe for a new planet, which astronomers still have yet to prove or disprove.

A great number of scientists are convinced it must exist, and so experiments and observations are still being made. Who knows, there could be a new character introduced to "The Desperate Houseplanets" any day now!