Dark spot covering half the eye (pic included)

I just adopted Burlington, an adult (not sure, they said 4-5 years possible) male Maine Coon, from an animal shelter. I'm attaching a picture of his right eye. It is not irritated or weepy, and he doesn't close it or rub at it. It doesn't appear to bother him at all. When I shine a light in his eye, it reacts, but not as much as the other eye does. They said he came in like that and it doesn't bother him, but looking online has me concerned, but I can't find PICTURES of these conditions to see if it looks like what he has.

I'm going to call the vet tomorrow, but I am half worried about that as I will have to go to a new vet since I've moved and my previous vet of 11 years is just too far away to use anymore. So I'm going into this not being sure I can trust the vet, having no previous history with him. The one I'm going to call has a bunch of positive reviews online and no negative ones, but still...

So has anyone seen this before?

I have never seen this before, so unfortunately I have no advice. I hope you get it figured out!

I have volunteered at rescues and handled literally a couple thousand cats in my life, and I've never seen it either! I'm really hoping it's nothing.

Hopefully just iris nevi (freckles)? Is that a velvety patch on the cornea?

No, it doesn't look velvety. The whole surface of the eye looks normal, nice and smooth and shiny.

From the picture, it looks like 1/2 the eye is a solid brown, and the other half has a lot of "freckles" in it?  Is that correct?  I'd skip the regular Vet and go see a Cat Eye specialist.  These things can turn into iris melanoma, that's why it's best to have an animal eye care specialist check them out.  Just my thoughts.

The vet wasn't worried about it today when we went in to see about his URI (I'll comment on my original post, but he was admitted for IV antibiotics and fluids). I'm not even sure we have an animal eye specialist around here. I will look into it further after I get him healthy again.

My cat sue who is 4.she has some thing very similar to your cat.my vet said she had eye infection when she was a kitten.her mum was a barn cat.the vet didnt seem concerned about it. i think she is blind in most of that eye.sometimes one pupil looks bigger than the other.i dont think it will do any harm.she is very happy and content.