When God made man He had two primary tasks for this man. We read from Genesis 1:26-28 that man was to be in the image and likeness of God to display Him and have interaction with Him, and man was to subdue and then rule the earth with its environs of sea and sky. The Kingdom pertains to this second task. To have to "subdue" means that illegal and unlawful activities are in progress, and rule mean to institute a certain regime. A careful study of the whole Bible reveals a battle between man and angels for the rule of the earth.
A Kingdom is a sphere where a certain king is resident and is able to impose his laws. If the king is not resident then he must rule from exile - which is not possible if he has an adversary present. One Adam decided against God as King, and put his trust in an angel, he has been ruled by this angel. But God is not to be thwarted. His councils are immutable and He is omnipotent. Starting with Abraham, God started a process of recovery for man that will end in a Man - Jesus, and His followers, ruling the earth, sea and sky.
Jesus came from God to offer the Kingdom of Heaven to Israel. Note, it is not the Kingdom IN Heaven. There is no problem in heaven. The problem is on earth so when Christ sets up His Father's Kingdom ON EARTH it is called the "Kingdom OF Heaven". That is, the rules that apply in heaven will be enforced on earth.
Now, as stated above, a Kingdom is a sphere where a certain king resides and rules. While our Lord Jesus was on earth, He did not try to rule over men. He ruled over the principalities that empowered men. This is shown by Him casting out demons (Lk.11.20). When Israel refused this Kingdom, our Lord Jesus started building a new people with which to rule the earth - the Church. This takes time, and because it required a man to be born of God, to be an heir, Jesus retired to heaven bodily and sent His Spirit to effect the building of this new Kingdom People. Thus, the Kingdom now is ONLY where God's People by this new birth are present and obedient to the Holy Spirit. That is, the Kingdom is IN you if you are born again and obedient (Lk.17:20-21).
When the required number, and the required maturity is reached, God will put an end to the "time of the Gentiles". The Gentiles ruled from Adam to Joshua, and again from the Babylonian captivity till the Beast comes. The Beast of Revelation 13 and 17 will be the last Gentile king. Then, the Beast and his military power will be defeated, Satan, the motivating and energizing force will be bound and imprisoned, and the Gentile SUBDUED and RULED by Christ, and those of His disciples who are found worthy will rule with Him (Lk.19:17-19). This Kingdom will have no end. The first 1,000 years of it will be on the earth as we know it now, with summer and winter, sea and harvest time, and the remainder of it will be on a RENEWED earth (lit. Gk.) for ever.