다 means all things. 아는데 means that I know but. so when they are combined, it mean I know all but. I guess it could be used in a dispute or argument. ex) A : I think our government need to expand welfare policy to support low income families. B : 다 아는데, How(where) they can get that huge money? B surely know what A is saying, but considering reality, it is not that easy~

다 아는데 = i know but...