An earlier post linked to an article describing the theory by Indian scientist Dr. Godfrey Louis that the occurance of red rain in an Indian province during a two-month period in 2001 may have been of extraterrestrial origin. At first assumed to be caused by desert dust, closer examination revealed the particles were not dust; in fact, they were striking like biological cells in appearance, although tests revealed no DNA.
This theory has been published in the peer-reviewed journal Astrophysics and Space Science. This link takes you to a page with photos extracted from the paper as well as links to the full paper itself. I know that extraordinary claims require extraordinary proofs, but if nothing else this deserves a closer look.
This is my own observation, but I notice the shape of these cells very closely resemble human red blood cells, which by the way also do not contain DNA (white blood cells do).