An expedition is like a space lottery. You can have positive, neutral or negative outcomes all based on luck. Here are a few examples:
Positive outcome: Your expedition comes across some abandoned ships and bring them home, adding them to your fleet. The ships you find are usually the same type as the ones that find them, and roughly 50% of the number in the expedition fleet. I have personally seen an expedition of 150 HF's come back with an extra 75 HF's.
Neutral outcome: Your ships searched and searched but found nothing, returning home empty handed.
Negative outcome: Your expedition fleet has encountered a black hole! Your fleet is partially destroyed!
Hope this helps!
-Istalris-