Why do dvd's have swirls on them?

I wasn’t sure where to post this question so I figured this would be a safe bet. Either way I am curious as I heard different things. Why is it dvds (original ones, not blank media) have swirls on them. Like I said, I have heard different reasons why but never anything certain. My guess was always, that’s the way they came from the plant. However recently someone said it was from playing a disc often which I find hard to believe. Any help in this matter would be appreciated.

Can you be a bit more specific, or (even better) post a pic?

I’ll take a stab in the dark and guess that you’re referring to the stress marks in the plastic layer that protects the data layer in DVDs and CDs.

I tried taking a pic but it doesn’t come out too well when showing the surface of a disc.

You might be right about it being stress marks (though I’d like to hear more about this). It looks like they come from the factory this way. They look like faint almost water like stains / swirls if that makes sense. Usually it’s not a big deal but recently concerning a disc I may purchase, the seller said the disc has swirls which happen when a disc is played often which I found to be a bit odd. So now this whole thing has me curious as to what the deal is with them. I also heard this can happen from leaving a disc too long in the player. Like i said, now I’m just pretty curious as to what the deal is.

Assuming the disc has not been abused in some fashion causing some kind of delamination, what you may be seeing is an optical light interference pattern (constructive / destructive) caused by some of the incident light being reflected off the plastic (polycarbonate?) layer and the rest of the light that is transmitted through the plastic and being reflected from the metal layer containing the data.