I often get asked how do we know dark matter exists? Which is why I've made a video on all the observational evidence we have before.
But occasionally I'll get asked how do we know how much dark matter there is, which is a really fun question.
There's many different ways we can calculate this, including the ratio between normal (baryonic) and dark matter, but in this video I just wanted to highlight three different ways astrophysicists calculate this.