What is Olbers' Paradox?

Olbers' Paradox is a famous problem which baffled many scientists in the early 19th century. 

The German astronomer Olbers (and many of his colleagues) made assumptions that the universe was static, of infinite size and age, and had uniform density (in other words, it was homogenous). 

Olbers posed the idea that if these assumptions were true, then every line of sight in the sky would end on a star, and that looking in any direction one would see light. In other words, the night sky must be uniformly bright. This is obviously not true, which posed a big problem for astronomers at the time. 

Olbers' Paradox was solved in the early 20th century by Edwin Hubble, who proved that Olbers and his peers had made incorrect assumptions. Hubble showed that the universe was of finite size and was expanding through his famous law.