'Nature abhors a vacuum' - this was apparently first stated by the Greek physicist and philosopher Parmenides around 485BC. Others including Aristotle have been credited with using the expression. In physics it means that if you remove everything, then something has to fill the space. A vacuum is always in tension with its surroundings. That is helpful as we use a vacuum cleaner to suck up dirt.
 
Jesus was talking to religious leaders about their practice of exorcism; commanding an unclean/impure/evil spirit to leave a person or place. He did not question the existence of evil spirits, indeed He would cast them out of people. Nor did He challenge the need for their authority to be taken away, or the peace which comes to a demonised individual when it leaves. But He made His hearers consider what happens to that evil spirit that is commanded to go.
 
Jesus' explanation is simple. The 'house' is a person's soul. If the evil spirit goes, there is no reason why it will not return unless the 'house' is filled with a stronger spirit. The 'house' has been made clean and there is even more room for evil, allowing other wicked spirits to also take up residence. The point which Jesus was making was that although the Jewish exorcists might be able to send some demons way, they had nothing to offer which would stop them returning. That is why people are so vulnerable if they try to get rid of darkness without receiving the light of Christ.
 
By contrast, Jesus promised His Holy Spirit to fill the hearts which He had healed (John 14:16-18). He does not remove Satan's influence and leave a spiritual vacuum. When Jesus deals with the oppression of Satan in someone's life He also promises to fill them with His Holy Spirit. Indeed if the Spirit of God is not present, a person cannot claim to belong to Christ (Romans 8:9). So the intervention of Jesus has two significant effects: He removes the authority of the 'Prince of this world' when sins are forgiven; and replaces it by the powerful presence of the Holy Spirit when the person is born again (Romans 8:15-16). The 19th century Scottish pastor Thomas Chalmers wrote about 'The Expulsive Power of a New Affection', he meant that when Jesus Christ is welcomed, Satan's agents have to go away and never return. That is such a simple message to give, "Welcome Jesus to stay".