There are several reasons your camera might not take a photo in low light.

First, it may be too dark for the autofocus system to work. Most cameras have AF systems which work down to about EV -1, which is dim ambient light. (Your eyes will adjust and be able to see relatively well, but the camera won't work.) And, in turn, many cameras will refuse to take a picture without a focus lock. There are three solutions here:

1. Buy one of the relatively-few recent higher-end cameras with support for focus down to EV -3 (moonlight). This probably isn't a practical answer, as your current options are the Canon 6D at $2000 or the Pentax K-5ii at $1100. It may be that in the future this will trickle-down to entry level cameras like the Rebel, but I expect it'll be kept as a higher-level feature for a while.
2. Use a focus-assist lamp. This feature is found on most add-on flashes (and can usually be used independently of the flash firing). Some cameras have a light built-in for this purpose, and others can strobe the built-in flash, which is very annoying but functional. Or, you could use a flashlight or laser pointer as an ad hoc solution.
3. Put the camera in manual focus mode and do it yourself.

Second, it may be too dark for the longest shutter speed allowed by your camera's auto mode. Options here are:

1. If you haven't already increased the ISO, do that. (On some cameras, auto-ISO will not go as high as setting it manually will.)
2. Use manual mode to set a very long exposure, ideally with a tripod.
3. Use the EV compensation to tell the camera you want an underexposed, dark image.
4. Add light, with a flash, by flipping a light switch, or waiting until dawn.