How could it be explained that living humans have yet to see heaven despite it being depicted as amongst the clouds, when they have technology to see vast distances in to space and have ventured out as far as the moon?
In my story there is a Heaven where people go when they pass, even though a large proportion of people do not believe it exists.
Heaven is 'above' Earth - if you move in the fourth dimension.
We haven't seen heaven because we're only capable of moving and seeing in three physical dimensions, but other three dimensional universes exist, separated from ours by some distance in a fourth dimension.
This fourth dimension has a gravity-like force which attracts a soul based on its goodness. It is ordinarily too weak to pull us from the material world, but once we die and our souls leave our bodies, good souls are attracted to heaven, and evil souls repulsed into a region we call hell. We refer to the regions close to the material plane as 'limbo'.
Some souls manage to remain truly neural, and remain in the material soul, where they can interact to some degree with reality. We call these souls ghosts.
Heaven is in the distant future, when we have mastered time travel and use a machine to take snapshots of historically significant human brains at the second before their death. Capturing every macro and quantum reaction going on in that moment from long ago, the machine draws the information forward in time to a Utopian city where a freshly manufactured and youthful body is waiting to receive it as a bootstrap for its own consciousness.
Since the information includes every memory, attitude and emotion that was going on when the snapshot was taken, right down to the source's current incomplete thought, the newly booted consciousness doesn't even notice that the hardware of its mind has been changed.
To that new consciousness, the only confusion comes from the sudden change in what their senses are telling them. They instantly realize that they are in a new and different place, and having been so close to death just a moment earlier, they come to the only obvious conclusion...
"Wow, I've died and gone to heaven!"
It depends on how much you want to hand wave reality. If heaven does actually exist, you probably have quite a bit of leeway to do so. With that in mind, there are a few possibilities:
Heaven is simply on a different “plane” of existence. Whether or not you want to consider this to be a different dimension depends on how seriously you want to try and explain it.
Heaven is a parallel or even alternate universe. Perhaps death is a transition from one to the other, with the “heaven” universe having whatever whimsical components you’re interested in.
Heaven is actually there in the clouds, but we are incapable of seeing or interacting with it. This gets a little more complicated to explain without hand waved magic, but the location is easier to mentally grasp.
The key, more generally, is to either a) conjure up a location that we physically cannot see / interact with or b) use something akin to magic.
