A good summary of the state of scientific knowledge about witchcraft can be found here:
Scientifically, the existence of magical powers and witchcraft are generally believed to lack credence and to be unsupported by high quality experimental testing, although individual witchcraft practices and effects may be open to scientific explanation or explained via mentalism and psychology.
Interestingly, in a 2005 Gallup poll, about 21% of people in the USA believed in witches: