Just like this is a visible world, there is an unseen world. If someone has a clean heart, God gives him/her access to the unseen world. Holy people, good-hearted people can go back and forth in both worlds. Satan of really high caliber can also get into the unseen world, but upto a limit
The unseen world is enormous, there are blessings and mysteries and miracles.
Modern science cannot explain these things, because it forms its basis on visible things.