What is the evidence for life existing on Europa?

There is no evidence at all...at least not yet.

We only have a long list of factors that may be in the right ranges for some kind of interesting organic chemistry to take place.

For instance, it has a liquid ocean under its ice crust because we can detect the magnetic field produced by the salty currents flowing in its interior. Also, it is likely that this ocean is mineral-rich and probably loaded with organic molecules because most icy bodies derive from cometary material.

But because all of this action is miles below the crust of Europa, we have no evidence on the surface that the interior has living systems. We will have to wait a few decades before NASA launches a probe to Europa to burrow through its 5 to 10 mile thick icy crust to study this ocean.