I can only guess what it's like in Heaven but, as far where Heaven is - Heaven exists outside of the Universe, outside of time, space, and matter in eternity, as God is.

This answer sounds a little "iffy", as Heaven IS a place (i.e. it takes up definite 'space'), material beings DO inhabit it, etc.

As for the original post, how can mortal beings still in via tell you about a world they have NEVER SEEN?  This is like asking a man who has never been to France to tell you all about France - NOT happening.

That's true, material beings do inhabit heaven. You are correct.

I would assume that Heaven is outside of the universe through, because if there is timelessness in Heaven then it would therefore exist outside of time. It just doesn't seem "right" that Heaven exists within the universe, rather than outside of it. But, I'm open to hearing different views on the matter.

I'll add my two cents.

Take a piece of paper and draw a line. The line represents time as we know it. The space around that line represents everything else, whatever that may be. When we die we go to this space, I cannot tell you whether or not it in itself has space, but it might be plausible. As had been said before, material beings have to occupy this area, so it could be plausible that they would need air to breathe, so oxygen can access this place. Then again, everyone who is there with a material body is there by supernatural authority, so who knows.
But like I said, once you leave the line and assuming you made it to heaven, you are far more intimate with God than the very saints who still live on Earth. That means you have access to all of knowledge, since God is Knowledge. You would be aware of everything that has or ever will happen on Earth. All of eternity is condensed into one infinite moment. Every sin that you commited and confessed will become a ruby on a crown greater than that of kings, as some saints have said. Take the happiest moment of your life, perhaps it was the birth of a newborn child, winning the lottery, receiving your first communion, whatever, and multiply that by a thousandfold. That is the happiness of heaven, except this happiness never wanes or fades, but it also never gets greater; you earned the happiness you obtained while here on Earth, but even to make it to heaven is a feat that few individuals will be privelaged to in any case. It's also interesting to note that there was once a saint that had the privelage of seeing the brightest light that the universe could ever conceive of producing; brighter than the brightest sun. It was said that not even that could equal a fraction of a single ray that was God or Heaven; for it would have blinded any mortal who looked at it, even through closed eyelids.
I imagine also that Heaven is going to be very big; for it encompasses time and all that is earth (unless we make room for Hell as well). So at the end of time, when we all receive our ressurected bodies, we will not have a care in the world and will be able to play and roam for all eternity, with all the children that have ever been.
Ah yes, that's another thing is worth mentioning. Once our bodies are ressurected, we will have absolute command over them. We could make them young again. We could choose to be children, or elderly. We could fly, we could go through walls, anything at all. Who knows, perhaps we could make our own materialistic things there in heaven, like a baseball. I see no reason why we could not, once we get our bodies back.
But yea, those are a few things about heaven that are worth thinking about. Just wait another 60 or so years, and all of that can be yours.

I found this on CreationWiki.org:

Can eternity have time and still be eternal?

Some would argue that the first 6 days of creation occurred in what is considered eternity. This is because the first sin did not happen until the creation was over. So what needs to be understood is how creation in eternity affects how creation had to be done.

We relate time with age, and death, because that is all that we have known. But God's word says that death was not so until man sinned, and sin did not come into the world until the creation was finished. Which also means that death was not a process of creation. And death requires you to age to be able to reach that goal which is the end of life. So death being a direct process of aging, means that aging was not a part of the creation either. So knowing that the 6 days of creation were done in 24 hour days. And because sin had not caused death yet. Means that the time during creation was eternal.

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"But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day." 2Peter 3:8 (KJV)

Heaven and earth are being compared in the verse above. Time here cannot be compared to a place that does not contain time. And next verse actually shows there is time that can be measured in eternity.

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"And when he had opened the seventh seal, there was silence in heaven about the space of half an hour." Revelation 8:1 (KJV)

A half an hour does not exist where time does not exist.

What makes time eternal in Heaven and not here?

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"But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day." 2Peter 3:8 (KJV)

This verse is a formulation for time dilation. Time dilation is where the faster you travel or move, the slower time becomes. And because of this, there is a speed in which the "effects of time" (aging) stops. So the formula goes like this: 1 day x 365 = 1 year. 1,000 years x 365 = 365,000 years. So 1 year compared to 365,000 years means that eternity (being with God) works 365,000 times faster than our time does.

Side note: The term "time being faster" is actually where the "beings in that eternal realm move faster". Time in all three realms (heaven, hell and where we are) are in sync.

This would explain:

   1. Why we cannot see God or anything in that eternal realm. We cannot see beings that move 365,000 times faster than us.
   2. How God can be everywhere at once.
   3. How God can know the past, present and future. Because they combine to become one (Alpha-Omega).

Alpha-Omega means that the time-line in eternity is just one big time-line that can be seen from beginning to end at the same time (all of time can be seen at one glance). Why? In the phrase "Alpha-Omega", notice that there is no "present time". Which means the beings in eternity are always moving through time. Eternity is all there is in time (beginning to end) bottled up to where any being there can be any where in time they want. So basically: Time - "present time" = time that moves 365,000 times faster than normal time. Time that moves 365,000 times faster = time - the effects of time (aging). Time - aging = eternity. And of course it has to be sinless to be Heaven.

What makes time eternal in Hell?

Every thing is the same as the above situation, except Hell contains sin, and Evil. So eternal Heaven is all the above plus purity (sinless and all that is good). Hell is all the above plus impurity (sin and evil). And neither one can exist inside of the other.

What determines which eternity we will end up in?

Because we are "all" born into sin, we have to "change" in order to fit into the laws that govern the eternal Heaven. If we choose not to change, then because of sin we will go to the eternity realm that allows sin and evil to exist through it's laws. So if a person prefers to sin and refuses to change, then there is only one eternity realm that allows this persons choice (Hell). If a person "chooses to change" and makes the covenant with the being (Christ) in Heaven who can provide this change through salvation. Then a person will end up where the sin-less laws govern that eternity (In Heaven).