The universe can be described by a list of particle species, and how strongly each species interacts with each of the others. For example, the electric charge of the electron, e, tells us how strongly the electron interacts with the photon. Put differently, imagining a photon flying past an electron, e tells us how often that photon will bump into the electron.
Some of the particles on this list are shy, they very rarely interact with the others. An example is the neutrino. Roughly a hundred trillion neutrinos emitted by the sun pass through your body every second. You don’t notice it because it only extremely rarely interacts with what you’re made of, i.e., the electrons, protons and neutrons that make up the atoms and molecules that make up you. It may very occasionally bump into an electron in one of your atoms, making it do a little wobble, but this happens so rarely that it has an imperceptible overall effect.
What else could be going through you right now? Maybe there are other things we haven’t discovered yet, because they’re so difficult to detect. The following is my attempt at an answer.