How do scientists know dark energy exists? a
We can only detect it by observing the effect of gravity on visible objects.
b
Because the universe we can see is only 4% of what the universe is actually made of.
c
Because it behaves like the opposite of gravity.
d
The observation that all galaxies seem to be moving apart from each other at a faster rate than in the past.

Scientists believe that dark matter may account for the unexplained motions of stars within galaxies. Computers play an important role in the search for dark matter information.

Explanation:

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the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

the ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (uvr) from the sun, protecting life on earth from its potentially harmful effects. when the sky is clear, there is an inverse relationship between stratospheric ozone and solar uvr measured at the earth's surface.