The Science

Gravity & the Missing Mass Problem

Einstein’s theory of General Relativity tells us that mass and energy curve spacetime, and that curvature is what we experience as gravity. If we know how much matter is in a system, we can predict how fast things should orbit or fall.

When astronomers apply those rules to galaxies and galaxy clusters, the numbers don’t work. The visible matter accounts for only a fraction of the gravity we measure.

Galaxy rotation: when gravity is too strong

In a spiral galaxy, stars farther from the center should orbit more slowly, just as planets far from the Sun move more slowly. Instead, their speeds flatten out: the outer stars move almost as fast as the inner ones.

If we trust gravity, this means there must be a lot of additional mass spread throughout and around the galaxy. That extra mass does not emit light. Hence: dark matter.

Clusters and cosmic structure

Galaxies inside clusters zip around so quickly that they should fly apart, unless the cluster is held together by much more gravity than visible matter can provide. The same conclusion appears when we use gravitational lensing to weigh clusters: the spacetime curvature is stronger than visible matter alone can explain.

Two broad options

The missing-mass problem pushes us toward two big possibilities:

• There really is additional, invisible matter (dark matter), or
• Our description of gravity breaks down on galactic and larger scales.

Many researchers work on both fronts: hunting for dark matter particles, and exploring modified gravity or extra-dimensional models that change how gravity behaves without adding new particles.

How this ties into DarkShadows5

The fictional universe of Dark Shadows of the Fifth Dimension takes a geometric approach. Instead of treating dark matter as a new kind of particle, it imagines that:

Gravity is being shaped by massive structures in a higher-dimensional space.

To observers on Earth, those structures show up only through their pull on spacetime. To them, we are just a small ripple on a four-dimensional brane.

For a gentler introduction, visit What Is Dark Matter?. To see how higher dimensions come into play, continue to Fifth-Dimensional Models, or jump into the story from the main page.