Dark matter
From Srafopedia
Dark Matter is the latest theory as to why the universe holds itself together. The calculations that were given by Einstein showed that gravity alone has not enough force to keep all the stars, planets and galaxies in the universe floating together - over 95% of the mass in the universe is unaccounted for. Galaxies should fly apart under the far greater force of centrifuge. So, unseeable, untouchable, and undetectable Dark Matter was used as a convenient way of explaining how the universe manages to keep it together.
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Relevance in His Dark Materials
Dark Matter is called Sraf in the Mulefa world, is renamed to Shadow Particles in the world of Mary Malone, Will, and ourselves, and is most famously known as Dust in Lyra's world.
