Universe's dark energy-matter content. Diagram showing the proportions of the Universe made up of dark energy (70%) and matter (30%). The matter can be dark (29%) or visible (1%). The visible matter (the Universe's luminous stars) is not enough to explain the universe's observed expansion rate. This missing mass can be accounted for by MACHOs (massive astrophysical compact halo objects) such as brown dwarf stars, or elementary particles like WIMPS (weakly-interacting massive particles). MACHOs are baryonic matter (containing protons and neutrons) and WIMPS are non-baryonic. Dark energy is a relatively unknown quantity that is thought to be driving the Universe's expansion.