Darkness is a matter of exposure. You cannot use the flash because players are too far for it to reach and, if it would, it would probably be disturbing to them.

While you can increase exposure or manually set it to get the desired brightness, this will force you to either increase ISO or decrease shutter-speed. Higher ISO means more noise and slower shutter-speed means blurry movement, which is usually undesirable for sports in general, other than for some artistic shots depicting movement.

What you can do is get a better lens. Mostly a brighter lens. Towards the telephoto end of the zoom, the lens you have has a dim F/5.6 aperture. Instead look for at least an F/4 lens, even F/2.8 if you can afford it. This will let in much more light, 2X and 4X respectively, and let you keep a high shutter-speed while getting a better exposure.

Canon has many prime lenses from 200mm to 400mm with F/2.8 or F/4 maximum aperture, although if you are shooting fast movement, a zoom makes it much easier to frame. Their 200-400mm F/4L is fantastic. You can also consider a third-party lens like the Sigma S 120-300mm F/2.8 DG OS HSM. It should be very suitable to your situation given its range and bright aperture.