Why isn't the universe homogeneous?

If the Big Bang created the universe in a single event then matter would be distributed uniformly. This would make star and galaxy formation impossible. Are there any theories on how this happened?

Yes. Look up Alan Guth and inflation.

Quantum fluctuations.

Gravity acts as a positive feedback to amplify fluctuations. If one part of the universe is slightly denser, that attracts more matter and gets even more dense.