The photo taken by my Canon 60D today is a little dark. Is the lens dirty or is the machine faulty?

Turn off the lights indoors. When the light is dim, take off the lens and open the M file 10 second for exposure. During exposure, blow the sensor with air, and then take photos outdoors normally to see if the black spots are still there.

If not, take it to the store and clean it up. If you have no experience, don't move yourself. The sensor is the core of the camera. If you hurt it, the camera will be useless.