When we think of a "kingdom", we usually think of a place… but the kingdom of God is more like a force for change that works within people than a physical location. In Matthew 13: 31-33, Jesus explains this by relating the kingdom of God to a mustard seed or yeast – something small and subtle, but which has the ability to slowly create enormous change.
Like an acorn that holds the power to cover the entire planet with trees given enough time, the kingdom of God works within us through a million small changes, rather than an explosion of growth. The power of the kingdom should not be underestimated however. In the same way that a mustard seed can someday grow into a plant capable of breaking through concrete, God's power within us is enough to overcome the immense difficulties of life. And in the same way that yeast works its way gradually through a whole lump of dough, the power of God within us can create drastic (positive) change within us as we gradually transition towards the person Jesus created us to be.