If you look at all your experience since you are born, it all happened in your mind, exclusively. The faces of your parents, your first girlfriend, your job, swimming, eating all sums up to senses which all appear in your mind/conciousness. So it is not only that conciousness exists, it is what everything you have ever known is made of. You can claim you experience something that is outside of you, but there is no proof for that. There will never be a proof. A proof would happen in your mind as well. The thoughts of 'outside of mind' will also happen in your mind. Based on experience conciousness is the only thing that really exists.