Pro
Aliens might exist because there is some evidence which proves their existence. One of them could be the strange apparitions on the sky which were on the news as well.
Another proof for aliens' existence are those people which are saying about strange things that happened to them, which might be true.
Con
Aliens might not exist as if they ever existed the scientists probably would have discovered them by using the space telescopes or other equipment.
Also those evidence found for aliens' existence they are not explained already, so they might or might not be true, it would depend on people.
