In many texts, therefore, the “heavens” stand for God himself and his sovereign position. His throne is in the heavens, that is, in the spirit realm over which he also rules. (Ps 103:19-21; 2Ch 20:6; Mt 23:22; Ac 7:49) From his supreme or ultimate position, Jehovah, in effect, ‘looks down’ upon the physical heavens and earth (Ps 14:2; 102:19; 113:6), and from this lofty position also speaks, answers petitions, and renders judgment. (1Ki 8:49; Ps 2:4-6; 76:8; Mt 3:17) So we read that Hezekiah and Isaiah, in the face of a grave threat, “kept praying . . . and crying to the heavens for aid.” (2Ch 32:20; compare 2Ch 30:27.) Jesus, too, used the heavens as representing God when asking the religious leaders whether the source of John’s baptism was “from heaven or from men.” (Mt 21:25; compare Joh 3:27.) The prodigal son confessed to having sinned “against heaven” as well as against his own father. (Lu 15:18, 21) “The kingdom of the heavens,” then, means not merely that it is based in and rules from the spiritual heavens but also that it is “the kingdom of God.”—Da 2:44; Mt 4:17; 21:43; 2Ti 4:18.
It's unseen (like electricity), but it's real, and will be revealed when God is good and ready... From it's description, it is a marvelous, wonderful, peaceful place, and I'm sure looking forward to seeing it one day. I don't know WHERE it is right now - maybe it's that 4th dimension that some say exist. Don't know. But just because my puny little mind doesn't know it all, doesn't mean it's not true. And I'm so glad it's true!
Heaven is the kingdom of God, which was opened for us by the merits of Christ's death on the cross. Heaven is the place of perfect happiness, where the souls of the just see the face of God even before the resurrection of their bodies. Heaven is the eternal community of love with the Father, Son, and Holy Spirit, Mary and the angels, and all the saints. Where is heaven? It is not on earth. It is "heaven." Strictly speaking, heaven is outside of space and time, it is "nowhere." Though we popularly speak of heaven as being "up" (somewhere above the clouds, in outer space or beyond the universe) and hell as being "down" (below the surface of the earth where there is a lot of fire and volcanic activity), these are just metaphors. No reasonable person actually believes that if we had the right kind of spacecraft and traveled far enough into outer space that we would find heaven. Nor would they believe that if we just dug in the right spot on earth and went down deep enough, that we would find hell. (My answer continues in the comments)
Heaven is not confined to one locality marked off by boundaries that can be seen or measured. It transcends the confines of time/space dimensions. Perhaps that is part of what Scripture means when it states that God inhabits eternity (Isa. 57:15). His dwelling place--heaven--is not subject to the normal limitations of finite dimensions.