Gravitational Lensing

Abstract

How can you tell something is there when you can’t see it? Usually there are indications: tracks in the snow tell you that an animal must have passed through. The smell of bacon in the morning promises breakfast deliciousness, even if you are still in bed and can’t see it yet. And if you look outside and see splashing puddles of water in the streets, you know it is raining, even if the falling droplets of water themselves are too small to make out. This is a bit how it is with dark matter: One cannot see it directly, but there are several indications that something must be there in the universe, even if it is invisible.