Sports mode tells the camera you need a fast shutter speed to freeze action. A fast shutter does not let much light in though.
If you were zoomed in, that compounds the shortage of light, as the variable aperture also constricts you as to the size of the opening on the telephoto end of the zoom.
It would be tough to shoot indoor sports in sports mode. Outdoors in daytime you would be fine.
I would get out of the Sports mode and try to take more control of the exposure. Try Tv (Time Value, a/k/a shutter priority) mode. That lets you set the shutter speed while the camera handles the other variables. In Sports mode the camera might set TOO fast a shutter, and thus deny you some light you could have used. If you can learn what is the minimum shutter speed to freeze a certain kind of action you can set that speed and not faster. Something like baseball or football might only need 1/400th of a second speed to freeze the action. Something faster like basketball or hockey might need 1/640th of a second. Experiment and see.
You can also get by with a bit longer/slower shutter by "panning" the camera. With practice you learn to smoothly track the moving subject so the majority of the subject (face/torso or chassis/windshield) is not moving much in relation to the camera, and is therefore in focus, and only the fastest moving parts are blurred from motion (feet/hands or wheels/tires).
Scott
Canon 5d mk 4, Canon 6D, EF 70-200mm L f/2.8 IS mk2; EF 16-35 f/2.8 L mk. III; Sigma 35mm f/1.4 "Art" EF 100mm f/2.8L Macro; EF 85mm f/1.8; EF 1.4x extender mk. 3; EF 24-105 f/4 L; EF-S 17-55mm f/2.8 IS; 3x Phottix Mitros+ speedlites
Why do so many people say "FER-tographer"? Do they take "fertographs"?