Bright and dark spots on brand new iMac

Take it back and ask for another new one. If the seller refuses or wants to keep and repair it,

then ask for a full refund using Apples 14 day money back guaranty.

see > Returns & Refunds - Shopping Help - Apple

That looks very much like the "searchlight" problem I had on my 2016 MacBook Pro (if it shows alternating light and dark areas across the entire bottom of the screen. See: 13-inch MacBook Pro Display Backlight Service Program - Apple Support

I've never heard of it on an iMac but my advice would be to return it immediately if you are withing Apple's 14 day return policy (or if you bought it from an Authorized Reseller and if you are within their return policy).

Edit- I concur with FoxFith. I hit send a couple minutes later.

If you just purchased the iMac and it's within the return window (typically 14 days I believe when purchased from Apple) then I would consider returning the iMac for exchange if the bright and dark spots are visible when using the system. I am able to see the issue in the picture you provided.

If the system is outside of the new product return window then definitely contact Apple to report the issue, and then schedule an appointment to have it evaluated either at an Apple retail store or an authorized service provider.

Out of curiosity, does the issue get better or worse depending on what image you are displaying? Can you see it when displaying a white email window? How about a darker image?

You're welcome! Yea' at least you spotted it right off...

That's great news! You're welcome!