Is this unreal?

Does the world really exist? Previously I posted about how Karl Barth considered that the existence of the world was a contestable hypothesis. He went further stating

the supreme problem of theology is not the existence of God, as natural theology supposes, but the independent existence of creaturely reality.

Barth is not alone in his concern that the existence of the world is a contestable hypothesis. Immanuel Kant considered that

‘it always remains a scandal of philosophy and universal human reason that the existence of things outside us ... should have to be assumed merely on faith, and that if it occurs to anyone to doubt it, we should be unable to answer him with a satisfactory proof’. 

The response of Martin Heidegger was even more interesting:

The 'scandal of philosophy' is not that this proof has yet to be given, but that such proofs are expected and attempted again and again.