In my science class book I was just reading about particles and antiparticles.
I'll just quote a few parts, "Every type of particle in nature must also have an alter ego, an antiparticle........One fascinating property of particle-antiparticles pairs is that if you bring such a pair together, the two particles will annihilate each other......when the pair annihilates, the mass of the two particles is converted into energy which is carried away by a pair of photons.....THE REVERSE is also possible, two photons may collide to produce a particle and it's antiparticle-by a process called pair production.
So assuming spirit creatures are made of energy, there is a scientific explanation of how energy can become physical mass, and physical mass to energy. Whether you believe they exist or not, I thought that was an interesting read and a way to explain how they could materialize and de-materialize.