I'll start by talking a little bit about physics. What do you think matter is? A table is solid in our perception and we haven't yet been able to walk through walls, so naturally it's reasonable to think that matter is solid stuff. But when you get down to the quantum level, 99.99% of the size of an atom is made up of empty space. Not only that but even the parts of the atom that have mass, we'll call them "quantic entities" may behave as a particle, or as a wave, depending on the experiment used to observe them. Heisenberg's uncertainty principle even states that there is no way to simultaneously know the momentum and the location of a particle, indicating a kind of non-locality of the particle. In other words, quantic entities seem to be a little bit everywhere all the time. And the famous experiment by Schrodinger about a cat in a box explains how in quantum physics, it's impossible to know whether a particle exists or not until it is being observed. So then to me what that says is that there is no way to prove that reality exists, unless we are somehow observing it, whether we observe with our sight or other senses, the perception is always a construct of the mind. Check out this video by quantum physicist Brian Greene to find out more about just how strange things get when you observe matter at the quantum level, which you can also find under "Science" on my media page.
But back to consciousness. Matter is both a particle and a wave, so really your brain is not only made up of matter, but is also entirely made up of waves.
Consciousness can then be both inside and outside of the perceived physical body as well.
Have you heard of quantum entanglement? To me quantum entanglement proves that matter is non-local, whether by its wave properties, or by something even stranger such as telepathic communication between the particles that allows each particle to know simultaneously the state of its particle pair. If a pair of particles can know each other's spin state whether it is immediately adjacent to its pair, or light years away, proves to me that there is definite interaction between what's going on in my mind, and yours. The different wave states observed in the brains of people in different states of consciousness (wakefulness, sleep, meditation/deep relaxation) may indicate that to connect to another person's consciousness, we may need to be in a similar brainwave state. And to me, this is even proof of a collective consciousness.
So what, then, is it that could be the initiating factor to all these waves of consciousness? Maybe it's God, or maybe it's your higher self - a sort of creative awareness, or maybe we are all avatars in an alien's computer game. The truth is out there. Or at least a truth we can all accept for the time being. Likely we will never understand the whole truth because it is possible that we are manifesting reality as we go. But we can still try.