Way back is the 4th or 5th century . Hundreds of years after the death of the witnesses to the cross,and the writing of the scriptures .
There is no contemporary witness report.
If God wished Mary to be as "venerated" as she is by Catholics, He certainly would have seen to it that the assumption was memorialized in the scriptures ...but God is silent on Mary after she received the Holy Spirit in the upper room
The Catholic church speculates on where her tomb is ...interesting seeing that they think her tomb is as empties as Christ's'
http://www.newadvent.org/cathen/14774a.htm
OK, take a deep breath. If we had the body of the woman who we call the Blessed Mother of God, do you think there's ANY chance we wouldn't be parading it all over the place? I mean come on, we've got places claiming to have all sorts of strange relics, if someone thought they had the body of the BVM (or maybe even thought they could get away with claiming they had the body...) we'd never hear the end of it. You wanna talk about Madonna on tour? That'd be a procession that would drive puritans to drink.
There's also the Church of the Holy Sepulcher which is all about an empty tomb as well.
It would be in His power for it to be so.....oh, wait she is. Is that because of double predestination?