Why is it prohibited to eat porc?

salam, I was with friends and one of them (not muslim) asked me : why, you muslims, do not eat porc ? Did you question yourself..Etc Well, I heard about some scientific research saying that the porc meat/fat contains some microscopic thigs that are not good for health, but I was not sure, plus science is so not "stable" thing, today something is good for health, tomorrow it's the worst thing ever.. So I didn't want to speak about what I do not know/master. I just said, sometime I do not ask questions, and I do not have enough knowledge, so I prefer to avoid to eat it till the day I will know why. So WHY ? why we do not eat porc, is it a clear verse in quran that explains why ? is there a known reason ? Thank you