With all the technological marvels we have today, it is tempting to think that we have all the answers. However, the reality of the situation is far different: many of the fundamental questions about the nature of the universe are still unanswered. Here are the things that keep cosmologists up at night. Dark Matter: Galaxies Are Weird We're all used to the fact that gravity keeps things together—it is gravity that keeps you affixed to the Earth, the Moon orbiting the Earth, and the Earth orbiting the Sun. We understand the interplay between mass and gravity intuitively. Unfortunately for astronomers, there is something weird about the gravity of galaxies. ESA/Hubble; NASA, Acknowledgement: Judy Schmidt Much like how gravity keeps the Earth in orbit around the Sun, gravity also keeps the stars in galaxies orbiting the center of the galaxy. Using a mix of observation and theory, astronomers are able to estimate just how much mass a galaxy contains and can estimate the gravitational forces exerted on stars. The trouble arises when you start looking at how fast stars orbit within a galaxy. Like the planets in our solar system, you would expect stars on the outer edge of a galaxy to orbit slower than the ones towards the center of the galaxy. However, that isn't what scientists see. Instead, stars orbit at about the same speed regardless of their distance from the galactic center. You might see this referred to as the "Galaxy Rotation Problem." NASA/European Space Agency Given the motion of the stars, scientists concluded there must be extra matter that we can't see distributed throughout galaxies. Scientists have dubbed this invisible extra matter "dark matter," and have been hunting for clues to explain it since it was first predicted. No one knows conclusively what dark matter is, but the most popular explanation is that dark matter is made up of weakly-interacting massive (massive relative to protons and neutrons anyway) particles, or WIMPs for short.…