From Mike Klymkowsky at PLOS blogs:
So where do (scientific) theories come from? Initially they are guesses about how the world works, as stated by Richard Feynman and the non-scientific nature of vague “theories”. Guesses that have evolved based on testing, confirmation, and where wrong – replacement with more and more accurate, logically well constructed and more widely applicable constructs – an example of the evolution of scientific knowledge. That is why ideas are cheap, they never had, or do not develop the disciplinary rigor necessary to become a theory. In fact, it often does not even matter, not really, to the people propounding these ideas whether they correspond to reality at all, as witness the stream of tweets from various politicians or the ease with which many apocalyptic predictions are replaced when they turn out to be incorrect. But how is the average person to identify the difference between a (more or less half-baked) idea and a scientific theory? Probably the easiest way is to ask, is the idea constantly being challenged, validated, and where necessary refined by both its proponents and its detractors. …
Another aspect of a scientific theory is whether it is fecund or sterile. Does its application lead to new observations that it can explain? In contrast, most ideas are dead ends. Consider the recent paper on the possibility that life arose outside of the Earth, a proposal known as pan-spermia (1) – “a very plausible conclusion – life may have been seeded here on Earth by life-bearing comets” – and recently tunneling into the web’s consciousness in stories implying the extra-terrestrial origins of cephalopods (see “no, octopuses don’t come from outer space.”) Unfortunately, no actual biological insights emerge from this idea (wild speculation), since it simply displaces the problem, if life did not arise here, how did it arise elsewhere? More.
So much cosmology seems to some of us to be merely ideas pretending to be theories, principally because it is unclear how they can be tested with respect to evidence, let alone “challenged, validated, and where necessary refined.”
See also: Panspermia (maybe life came from outer space) is back, in Progress in Biophysics and Molecular Biology
and
Question for multiverse theorists: To what can science appeal, if not evidence?