One of the Bible's greatest truths is that you and I weren't made for this life alone. We were made for eternity - an eternity with God in heaven forever. And down inside, most of us sense this, even if we don't know much about God. As the Bible says, "He has also set eternity in the hearts of men" (Ecclesiastes 3:11).
But how do we know heaven isn't something we just dreamed up (as you suggest)? One reason is because of God's promises. From one end of the Bible to the other, God tells us that we were created in His image, and because of this we have an immortal soul or spirit that can never die. The Psalmist declared, "Surely goodness and love will follow me all the days of my life, and I will dwell in the house of the Lord forever" (Psalm 23:6).
Most of all, however, we know heaven is real because of Jesus Christ. He came from heaven for one reason: to make it possible for us to go there. Because we are sinners, heaven's door is closed to us. But on the cross Jesus Christ took away all our sins, and now our sins can be forgiven and cleansed, and we can have the privilege of entering heaven.
But we also know heaven is real because Jesus Christ rose from the dead. This life is not all! Put your faith and trust in Christ today, for He alone is "the resurrection and the life. He who believes in me will live, even though he dies" (John 11:25).