The theory that a cross-transfer of evidence occurs whenever any person comes into contact with an object or another person is:

The theory is known as the Locard's exchange principle

It was created by a French forensic scientist called Edmond Locard who was something like a real life Sherlock Holmes. He pioneered modern forensics and many of his principles are still applied to this date, even more easily because of modern technology.

One splits the atoms and molecules  the other combines them