In low light, AF usually struggles more with lack of contrast than lack of exposure light. A few practical options can help:

• Use a Canon flash or transmitter with an AF-assist beam. You can often disable flash firing and still use the assist beam to help the camera lock focus.
• A Canon ST-E2-style transmitter can provide AF assist without the size of a full flash.
• Pre-focus on a spot with good light, detail, or contrast—such as a marked area on the floor—and wait for dancers to move into that zone.
• If your camera supports it, use a focus-trap approach: preselect the focus point/area and let the shutter fire when the subject reaches focus.
• Choose AF points that are most sensitive in low light, and aim them at areas with clear contrast or texture.

With fast lenses, the very shallow depth of field makes accurate focus harder, so combining AF assist with pre-focusing is often the most reliable solution for dancers in dim venues.