A: Why are going to Edgar's ?
B: Just because...
A: Wieso gehst du zu Edgar?
B: Nur weil...
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If the answer is "Just because.", without any explanation, you could say (as c.p. wrote):
This one might imply that there is no specific reason/motiviation (or that you don’t want to tell it):
A little bit cheeky, and often used if someone is asking too much, or if you see no reason why you should answer the question:
If you can’t (because you don’t exactly know or you are not allowed to) or don’t want to explain, you could say (it’s probably a bit funny):
Aus Gründen. [because reasons]