the screenshot window isn't closing and the rest of the screen is a bit darker. how to close it?

Check your Menu Bar for an icon of a hollow square inside a filled-in circle. If your Mac is screen recording, this will stop the recording. If you don't see the icon, try pressing the Escape (ESC) key in the top left corner of your keyboard. Pressing Escape should end any screen shot or screen recording in process.

I had the same problem. I was taking a screenshot of a frame image within a video in QuickTime. Screenshot for some reason got in a "capture" video recording mode. I had to open Screenshot app again and stop recording the video. That closed Screenshot completely.

from your description, you may have inadvertently started a screen recording instead of a screen shot. look at your menubar and see if there is a "stop button". if there is one, click it to stop the recording.

and for your further reference, please see this:

Did you already tap the "ESC" key on your keyboard?

If this does not work, bring up the task manager to relaunch the Finder.

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copied from: How to force an app to quit on your Mac - Apple Support

sorry, but "ESC" does not stop a screen recording.