There is one way, but you better be ready for some fighting!
You need to beat The Elite Four 50 times (Be careful. The Elite for will be stronger than the first time you beat them, so make sure your pokemon are stronger too), and then Fly/Surf to Mossdeep City. Once there, head over to the Space center and speak to the dude with the white coat. If he says "thats the 100th successful launch" (or something similar, but it will have to be the 100th), then He will offer to send you to the moon, for some exploring!
NOTE: I have not tried this. A mate of mine told he it was possible, as he had done it.
Hope this helped out!