The bottom of the cube is a 2. This is inferred based on 3 pieces of evidence: 1) Only even numbered sides are dark colored, and 2 is the only remaining even number that is not visible. 2) There are 3 light colored sides and 2 dark colored sides, following the pattern of an equal number of each. 3) The opposite sides that are visible add up to 7, and with the top being 5, the bottom would need to be a 2 to complete the pattern.