According to eyewitness accounts of 'aliens' we have a Universe teeming with extra terrestrial neighbours of all shapes, sizes, and colours. Yet despite the pictures, reports, testimonies, and video footage, we have no definitive proof of life anywhere else other than the rock we call the Earth. We've sent probes to other planets, but still nothing. ET is either good at hiding, doesn't exist, we're looking in the wrong places, or we haven't yet developed the technology to find alien life.
There are of course tantalising signs that planets may have once harboured life, such as the evidence of water on Mars. Water is fundamental to life as 'we know' it, but this isn't as satisfactory as finding proof. Chemical signatures such as the presence of methane have been detected, but these results are ambiguous as it's not just biological processes that produce the gas. Supposed fossils of alien microbial life are equally contentious.
There are many scientists who think that if there is life on another nearby planet, for example Mars, it will probably be microbial and exist just below the surface, where liquid water might be widespread.