What Is Dark Matter?

Astronomers use the term dark matter for whatever is providing extra gravity in the universe but doesn’t emit light. It’s not one specific substance we’ve already detected — it’s a placeholder name for a gravitational problem we can’t explain with visible matter alone.

Why we think it exists

When we measure how fast stars orbit in a galaxy, the outer stars move much faster than they should if only the visible stars and gas were present. The same is true for galaxies orbiting within clusters. Either:

• there is extra, invisible mass, or
• our understanding of gravity is incomplete.

Key evidence

1. Galaxy rotation curves: rotation speeds stay flat instead of dropping with distance, implying a large halo of unseen mass surrounding each galaxy.

2. Galaxy clusters: the way galaxies move inside clusters, and how the clusters bend background light, both demand far more mass than we can see.

3. Cosmic microwave background: tiny temperature patterns in the afterglow of the Big Bang are best explained if most of the universe’s matter is dark.

What could dark matter be?

Standard ideas focus on new kinds of particles: WIMPs, axions, or other exotic fields. But there are also more radical possibilities:

• gravity behaving differently on large scales,
• extra dimensions that let gravity leak between layers of reality,
• or dark matter as a geometric effect rather than a cloud of particles.

Where DarkShadows5 fits in

The fictional universe of Dark Shadows of the Fifth Dimension takes one of these speculative ideas seriously: that dark matter might be the shadow of structures living in a higher-dimensional space. In the story, Earth eventually becomes visible to one of those structures.