Or you can simply bracket your exposures. Auto-focus systems may also be adversely affected by low light levels. Some cameras focus better than others in low light, but there are a few tricks you can use to help any auto-focus system. If you are photographing a spotlighted act, for instance, keep the brightly lit subject centered in the focusing area of the viewfinder. If you don't want the subject in the center, you can use the focus lock to hold focus while you recompose.
Museums have fascinating displays which offer superb opportunities for existing-light photographs.
When the scene and displayed objects are lighted evenly, expose according to an overall meter reading. Otherwise get as close as possible to the subject to make your reading.