I agree with Disturbing.Making a game too dark is not scary.It's annoying.What is the point of playing an fps if you can't see what you are shooting at?
You can do a variety of things with lighting,design etc. that can make a level scary.
If the main generator went out wouldn't the back up generator fire up emergency lighting?
If all the lighting were out wouldn't the life support be out,too?
It would have to be a quick game with no oxygen.
My point in all of this is that too many people just make their level too dark because they think that makes it scary.That's taking the easy way out.
Level design,lighting,enemy placement and other things need to come into play as well.
It will take time for you to come up with a good scary game.Nobody designed a great game overnight.Keep working on it.