I think I came up with a way to fill in the empty spot.
I'm not sure why putting a shower curtain in would make the shower so dark, but what can I do? I added a wall and a cabinet with shelving, although the cabinet isn't quite the right sizing as I would've hoped for.
Since adding the cabinet, I figured the vanity needed a bit of a makeover; I changed it to the oak texture. Now it goes better with the cabinet/shelving.