!I!n!s!e!r!t! !n!b!e!t!w!e!e!n!

QuadR, 1 byte

Thanks to A__ for halving the byte count!

!

Try it online!

Replaces nothing with !

Python 3, 26 bytes

lambda s:s.replace('','!')

Try it online!

Retina 0.8.2, 2 bytes

!

Try it online! At last, a challenge where Retina has a built-in!