how do you install a dmg file

See? Even I can make one…

Do don't. It's a virtual disk. Double click on it to mount it. Then install what's on it. Run the installer if there is one, of not just drag the app to the Applications folder. Dismount the DMG. Done.

double-click it, or control-click open.

¿What .dmg file is this?

Where did you download the dmg from?

Well mate you got a dodgy one then.

FWIW, it's easy enough for someone to make an empty .dmg file with a black background. Why they would I couldn't say.

You've created a "black hole". 😉

Yes! lol