You can't run that fast, how do you know? :P
Maths and theories.
i told you he was a witch!
People like you would think that
But that's the problem. We have huge theories and models based upon nothing but guesses which are frequently confirmed wrong. For example, the big bang, dark energy, and dark matter are all just theories built upon mathematical models which are completely unconfirmed. They then create experiments to confirm these models which ignoring all experiments which confirm them invalid.
We really don't know very much about the universe. For example, the expanding universe theory can also be explained by plasma. Black holes can also be explained by plasma. A plasma universe doesn't require dark energy/matter. So on and so on.
This is what most astrophysicist metaphorically actually do: https://www.boredpanda.com/medieval-animal-paintings-that-dont-look-real-daniel-holland/
the expanding universe theory can also be explained by plasma.
It doesn't have to.
The Doppler Effect explains and proves it.
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