The search for dark matter

Physicists are very particular about balancing budgets. Energy charge and momentum all have to be conserved - and often money as well. Astronomers were therefore surprised and disturbed to learn in the 1930s that our own Milky Way galaxy behaved as if it contained more matter than could be seen with telescopes. This puzzling non-luminous matter became known as "dark matter" and we now know that over 90% of the matter in the entire universe is dark.