Belief in the unseen is a requisite in Islam and signifies belief in anything of which Allah the Almighty or His Messenger (peace and blessings of Allah be upon him) has informed us. Belief in the unseen is not comprehended by the intellect, reasoning or any form of analogy.
For matters relating to the Unseen are to be accepted without questioning as their true nature is known to Allah alone. Therefore, we believe in them as Allah informed us without qualifying them or likening them to anything. That is why Allah describes His obedient servants as those "who believe in the Unseen" (2:3), and so He made this belief a special quality of the believers, that is their belief in the Unseen of which Allah has informed them, for no one who informs of the Unseen knows better than Almighty Allah.
Taken from THIS IS ISLAM