Luckily we don't accumulate knowledge by counting things we are ignorant of as evidence. This is known as an
appeal to ignorance.
Considering the number of planets able to support life,(
drake equation says about 20 just in the milky way alone) it seems very very likely we are not alone, however we have absolutely no evidence of this, not a shred, also known a the
fermi paradox. To explain why we have such a contrast between high probability of alien life but no evidence of contact, some people have come up the concept of
the great filter.
Aliens would still be subject to the laws of physics, and considering to travel just one light year you would have to go 186,282 mps for an entire year, you start to get an idea of how unlikely it is that even an advanced civilization could travel to our planet. So although the possibility of other life seems quite high, the chance that they have achieved faster than light travel seems very low, and the idea that once they do overcome this obstacle they then travel to earth to mutilate cattle and leave functionless bits of metal in peoples feet, seems absurd.