A very interesting and difficult part of these kinds of questions is that what we look for has to be very carefully considered. If we look for just what we know

A very interesting and difficult part of these kinds of questions is that what we look for has to be very carefully considered. If we look for just what we know now, we might not find the evidence, even if it's there. One of my favorite examples of this is radio waves. Try telling someone in the 1600s about them; it seems unlikely they'd believe you--but yet they've existed forever (?) and e.g. lightning and other natural forces can generate them.