Why Does Dark Energy Dominate Outside Galaxies While Dark Matter is the Winner Inside Galaxies?

-- Ebrahim | August 3, 2020

Question:

Hello,

The idea of dark matter and dark energy is puzzling me. It is understood that dark matter acts like glue to hold galaxies together. On the other hand, dark energy is responsible for expansion of universe. Scientists say inside galaxies dark matter overcomes dark energy so gravity wins and keeps galaxies together but when it comes to intergalactic space, it is said that dark energy overcomes dark matter and wins the fight, so galaxies are moving away. How do you explain this contradiction? Why dark energy wins in intergalactic space but not inside the galaxies.
In other words, what makes dark matter overcomes dark energy in galaxies but loses in the space between galaxies.

Thank you,

-- Ebrahim

Answer:

The reason that Dark Matter plays a larger role in the motions of objects within galaxies while Dark Energy is the dominant force between galaxies is that the density of Dark Energy is very low (about 7×10^(-30) g/cm^3). Since the density of Dark Energy is much lower than the density of Dark Matter or ordinary matter, Dark Matter and ordinary matter dominate within galaxies. On the other hand, since Dark Energy is spread-out over the entire universe, it dominates the mass-energy of the universe on large size scales.

-- Jeff Mangum

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