Matter surrounds us day and night in all its forms—trees, houses, furniture, and even the air we breathe. But, according to physicists, the visible matter familiar to us may only account for approximately 20 percent of all material in the universe. According to the current theory, as much as 80 percent may be dark matter. This claim is based on several observations, one of which is that stars and galaxies rotate much faster than they would if there were only ‘normal’ matter present in the universe.
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Source: Phys.org