Back in 1997 I wrote a pioneering essay entitled "Life in Dark Solar Systems." The primary web address (URL) for this essay is http://astronomy-links.net/Dark_Life.html. Fifteen years ago there was little theory or evidence supporting wandering dark planets, and no technology supporting a census of exoplanets. However, I specialize in absurdist astronomy, so I took a stab in the dark at this astronomical ghost phenomenon, knowing that the essay would be ignored.
I hypothesized that, using extremophile models on Earth, it was likely that even so-called dark solar systems without a bright central star could support life on planets having sufficient internally generated heat. Today this thesis is so well assumed within the astronomical community as to be almost a cliche.
I wrote and published another essay in early 1983, shortly before the Pasteur Institute in France announced their discovery of the human immunodeficiency virus (HIV). I was the new editor of a publication in Santa Fe, and chose to dynamically describe what was then a rare disease called AIDS, for which there had been no scientific explanation. (The outgoing editor sharply criticized me for wasting space writing about something of no significance.) I wrote that AIDS is a viral disease, and I also described the mechanism for its latency, and how it could be treated. I called this model the seed and soil metaphor.
It would be fair to say now that all life is a "seed" that requires a "soil." How far early life on any planet advances depends on time and luck. Life can start with promise, only to be snuffed out by global disasters that radically alter the "soil." Early Earth may have had multiple episodes of life starting, only to be mostly snuffed out by cosmic collisions, global freezing, and other phenomena that could happen again.
Recent astronomical science has revealed the likelihood of "billions and billions" of Milky Way planets, often around red dwarfs, and some even floating alone in space, or near failed stars. See these sources: (1) http://www.wired.com/wiredscience/2011/05/cold-lonely-exoplanets/; (2) http://www.bbc.co.uk/news/science-environment-17532470; and (3) http://www.eso.org/sci/facilities/lasilla/instruments/harps/index.html.
One interesting source of runaway stars and planets could be the possible 25,000 small black holes orbiting the core of our Milky Way; but not the supermassive one itself. An article published in 2006 suggested that hypervelocity stars and their planets could be common. Here is the URL: http://www.newscientist.com/article/dn10020-puny-black-holes-can-eject-milky-ways-stars.html.
Which type of dark planets would be more likely to harbor some forms of primitive life? I suggest that the presence of comet bombardment (providing water and organics), along with the nutrient soup of dense space dust clouds, could be every bit as important as a body large enough to produce Earth-like surface temperatures.
Most dark planets probably don't have the lucky externalities necessary to support a biosphere, including planets previously within parent stellar systems that were violently cast outward by black holes. Although primitive life is likely spontaneous with enough of the right preconditions and ingredients, life itself is not ubiquitous; and advanced life is likely rare.
Even on Earth there are places so hostile to advanced life that little is found. When James Cameron recently descended alone over 35,000 feet to the deepest trench on Earth, he was looking for exotic creatures such as those found in other deep oceanic regions. He found none of the kind. There surely are many species of microorganisms at the bottom of the Mariana Trench, but not enough "good soil" there to nourish the "seeds" of larger life forms.
We in Earth's biosphere must speculate about highly evolved exolife, as it is hard enough to document independent microbial exolife on Solar System prospects, such as Europa. The existence of simple exolife beyond Earth is assumed to be likely by most 21st century theologians. Modern theology has progressed beyond the quaint Garden of Eden. Most church thinkers now embrace the idea of life sprouting up in many places of the universe, to the enhanced glory of God the Creator.
That is fine, but the real question people want answered is: "Where are the UFOs?"