What happened after Hitler found out that he wasn't the only one who hated the Jews?

Hitler did not "discover" one day that Anti-Semitism was common in Europe. Most Europeans were well-aware of Anti-Semitism that was prevalent in their societies at the age of maturity.

It is the same way that US Americans do not "discover" that racism (usually informal racism) is common in the USA. They are well-aware of the situation by the age of maturity.

When Hitler found enough Anti-Semites, he began to have political ambitions.