Are the pictures from your Canon Rebel DSLR coming out with the lower half of the frame black? When you take off the lens and push the main mirror up, does the small mirror below the main mirror not lift up completely? If so, your sub-mirror pin is broken and causing the darkness. There is a pin that functions as a hinge for the sub-mirror. When it breaks the mirror does not fully fold up into the main mirror and thus blocks some light to the CCD sensor. You can manually fold the sub-mirror into the main mirror, which will fix the problem at the cost of your autofocus functionality.
In a move of remarkable engineering Canon decided to make a critical high-stress pin out of plastic. Who would have guessed that a tiny plastic pin would break after being flexed 20,000 times? The only way to really fix the issue is to install a new metal pin, which Canon eventually did.