Ideally it should:
• Have zero electrical charge – so it can sneak past electrons and protons
• Be able to go deep inside stuff and not just measure the surface
• Scatter about the same from any type of atom
• Have a similar size to the spaces between atoms
• Act like a little compass to sense magnetic fields
Luckily, such a particle exists. It’s called a neutron.
And it’s great for seeing light little atoms like hydrogen, where they are and what they're up to.