Where is Heaven?
First, we understand that heaven is God’s heavenly realm now. The place where God is. We pray in the Lord’s prayer: “Our Father in heaven" (see Matt 6:9). So we think of heaven as a "spiritual dimension".
When the Russian cosmonaut Yuri Gagarin first went into orbit he famously said “I looked and looked but I didn't see God.” But Heaven, (where God dwells now) is not a part of the created physical universe. Rather it is a spiritual realm. We know from many parts of the scripture that God reigns from heaven and angelic beings serve and worship Him there. See Genesis 28:12, Mark 13:32, or Rev 19:4. Throughout the Bible this realm is only glimpsed when God chooses to reveal himself to his servants (see Isaiah 6:1 or Daniel 7:9-28).
But secondly the Bible speaks to us of the “new heavens and new earth" (see 2 Pet 3:13 or Rev 21:1-2). These passages refer to the time to come when God will renew the whole of His creation. The Spiritual realm of God will join with the physical realm of creation. God and man will be brought together again as it was in the beginning. This is the hope of the Christian – that God’s kingdom will come on earth as it is in heaven.