What do I mean with such a weird title?
Simply this- have you ever taken some astronomy pictures, with the intention to process them later only to discover you forgot to take some darks that night? Oh well! Here's a processing tip that seemed to work for me.
One night in August I put my digital camera on its tripod, and aimed in the region of Hercules. I was on vacation in a place called Magnetawan, Ontario and on this night I was trying to observe M13, which I had never seen before. I was feeling a little bewildered, didn't really know where it was, and was looking for the Keystone but unfamiliar with this part of the sky. Seeing so many more stars also made things difficult. So I thought I'd take a few pictures, planning to process them later to help me confirm that I was looking at the right spot. About a week later I looked at the pics and realized they were pretty bad... the camera can only go to maximum 16 seconds exposure and has a very low iso, probably something like 100, so the exposure was not sufficient. Worse, there seemed to be an artificial glow of light spreading diagonally from one corner of the image to the other.
WARNING: BAD IMAGES AHEAD. This post is about how to fix a problem, not necessarily to produce the finest quality images. What I end up with is sub-par compared to what some of you are doing! Oh well, you were warned!
Unprocessed image:
I could see a few specs here and there, but the gradient of light coming from the top corner made the image difficult to improve upon. I think this is called "Amp Glow". I forgot to take a dark image in order to subtract it. So, after thinking for a few weeks on what my options were, other than getting the camera out and actually taking some darks... thought I'd try using a copy of the same image to flatten itself. I made a duplicate of the image in memory. The trick was to remove the stars from this image, but leave the amp glow intact. I applied a median filter to it, carefully selecting the filter's aperture... so that the stars completely disappeared, but the gradient of artificial light remained. This now became my dark image, by subtracting it from the unfiltered version of the same image file. This seems to work well in solving the problem, so that I could process the image a bit more to show the stars.
Result-
Ok, not a "stellar" image 
Although this technique is not as good as using real darks, I hope you might find it useful to solve a problem some day.