Today the superclusters of galaxies are arranged in walls surrounding huge empty spaces called voids, giving the Universe the structure of foam. The following diagram is a simulation of its present structure, based on the previous simulated history.
Simulation of Universe today. Click for image credit.
You can see the walls made of clusters of galaxies and the voids between them. The simulation is not perfect but it gives some credibility to the dark matter theory.
Letís try to get a feeling for the size of the voids in the Universe today using the PennySystem model. If the Solar System were the size of a penny, then a typical void in the Universe would be about 4,000 km (about 2,500 miles) across, which is about the size of Australia, and large void would be about as large as the Earth.
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