I think one of the biggest reasons why people believe life on Earth is a 'lucky one-off' is that we don't see life on other planets. We appear to be alone in the universe. This gives support to the notion that life is arbitrary, without intent, without a plan. But could there be another explanation?
Limited perception is a common feature in Nature. For example, to you and me a field is a cacophony of insect noises, but to the grasshopper, who has a very narrow auditory range, a field is a silent lonely place in which he hears his own kind. This limited perception ensures his survival - for he can identify a mate or rival at distance without the interference of other insect noises.
We are well aware of auditory spectrums, visual spectrums and it is very possible that we live within a density spectrum, in which we only perceive that which exists within our spectrum.
We take for granted the fact that our visible spectral range represents a very small fraction of the full electromagnetic spectrum; and we take for granted the fact that our audible sound range represents a small part of the sound spectrum. We hear between 20-20,000 Hz. The bones of a dolphin's ear are almost identical to a human ear, but dolphins communicate at frequencies up to ten times the audible limit of the human ear. Bats communicate between 25-50kHz. Everyone has seen a dog or cat perk up their ears to a noise that completely evades us.
Science has discovered that we only perceive 4% of our universe. That means 96% we don't perceive! We know there is 'something out there' because of how the universe and bodies in the universe 'act'. The discovery of limited perception is important because it shows that matter acts like light and sound - there is a spectrum of perception and we only perceive that which lies within our range. Like grasshoppers in a field, our perception could be limited by Nature to ensure our survival.
There could be life on other planets in our solar system, on other planets in other solar systems that exist alongside us but within different density spectra. Our limited perception then becomes an identifying feature of evolution (which again is common in Nature) in that it acts like the shell of an egg - isolating us as we develop/evolve.
Edited by Vanessa, : as requested - to make myself better understood
The title of your post is about the possible abundance of life in the universe, but after the opening paragraph you talk only about limited ranges of perception. Could you add a few sentences of clarification to your post that make clear how you're connecting the two? For example, are you saying that there could be planets of dark matter harboring life?