The Kingdom of Messiah was at hand and to the Pharisees who asked Christ the question, He was definitely not saying that the Kingdom was within them. He was saying that because of their hardness of heart which resulted in their blindness, they could not see that the promised Messiah was now standing in their midst. He had already told them that they would not receive any further sign than His resurrection.
Obviously, when the King returns and the Kingdom actually comes to earth, it will be glaringly visible and observable:
Luk 17:24 For as the lightning that flashes out of one part under heaven shines to the other part under heaven, so also the Son of Man will be in His day.
Luk 17:25 But first He must suffer many things and be rejected by this generation.
Luk 21:25 "And there will be signs in the sun, in the moon, and in the stars; and on the earth distress of nations, with perplexity, the sea and the waves roaring;
Luk 21:26 men's hearts failing them from fear and the expectation of those things which are coming on the earth, for the powers of the heavens will be shaken.
Luk 21:27 Then they will see the Son of Man coming in a cloud with power and great glory.
Luk 21:28 Now when these things begin to happen, look up and lift up your heads, because your redemption draws near."
Mat 25:31 "When the Son of Man comes in His glory, and all the holy angels with Him, then He will sit on the throne of His glory.
Mat 25:32 All the nations will be gathered before Him, and He will separate them one from another, as a shepherd divides his sheep from the goats.