The term “kingdom” as used by the Jews often stressed the abstract idea of “reign” or “dominion”, not some geographical area surrounded by physical boundaries.  The Kingdom of Heaven is wherever the REIGN of God is manifested.

The Kingdom of God has always existed but was manifest on earth (again) with the coming of Christ which Daniel foretold, was proclaimed by John the Baptist, and was preached by Jesus.

The reign of God is expressly manifested today through Jesus Christ, through whom we have access:

But because of his great love for us, God, who is rich in mercy, made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved. And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— not by works, so that no one can boast. For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do. (Ephesians 2:4-10 NIV)

Therefore, my brothers and sisters, make every effort to confirm your calling and election. For if you do these things, you will never stumble, and you will receive a rich welcome into the eternal kingdom of our Lord and Savior Jesus Christ. (2 Peter 1:10, 11 NIV)