why pictures get darked at good light?

Well, since you've already reset the camera, I'd look at CanonUSA.com for a firmware update. While there, you can cruise the service advisories to see if there's an ongoing issue that's been addressed by Canon about it.

But it may also be shooting style. If you take the picture and there's a strong light source coming into the lens, then even on a bright day you will end up with an underexposed subject as the lens stops down, closing down the amount of light exposing the ccd chip. Also, you can shoot with flash, even in daylight if your subject is within the flash range. This will illuminate your subject and make it "pop." At the very least, always be sure the light is coming over your shoulder and onto your subject.

But if that's been eliminated as a culprit as well, you may need to have the camera serviced.