What do scientists means when they say that something "exists"? In this video, I explain that really we can only confidently say that our theories describe observation. When we say that something exists, we mean it is useful to describe our observations well. Elementary particles, gravitational waves, black holes, even space and time themselves exist in this very sense: They are useful ingredients of the mathematical theories that we use to make predictions which agree with observation. Support me on Patreon: https://www.patreon.com/Sabine?fan_landing=true