Occam's razor is a principle commonly stated as "Entities must not be multiplied beyond necessity". When several theories are able to explain the same observations, Occam's razor suggests the simpler one is preferable. It must be noted that Occam's razor is a requirement for the simplicity of theories, not for the size of the systems described by those theories. For example, the immensity of the Universe isn't at odds with the principle of Occam's razor.
Occam's razor can be grounded in the conjunction rule of probability theory: the conjunction A and B is necessarily less (or equally, in the case of logical equivalence) probable than the A alone; every detail you tack onto your story drives the probability down. Occam's razor has also joking been referred to as "Solomonoff's lightsaber", in reference to Solomonoff induction.