In the 1970s, Vera Rubin and Kent Ford measured how spiral galaxies, including Andromeda, rotate. The surprise: stars far from the centre orbit just as fast as those near it, when they should slow down, like the outer planets around the Sun. The most widely accepted explanation is an invisible mass wrapped around each galaxy. It remains one of the strongest clues for dark matter.
A large share of Earth's water molecules formed in the interstellar cloud that existed before the Sun was even born, more than 4.6 billion years ago. You are literally drinking a cosmic fossil.
By day, the Martian sky looks butterscotch because of fine dust hanging in the air. But at dusk, those same grains scatter blue light forward, creating a bluish glow around the Sun. Rovers have photographed these blue sunsets: the exact reverse of what we see on Earth.