We only know it exists because of its effect on the expansion of the Universe.

She looked down the road for him, the pain in her heart swelling up again.

When they are found, doctors pass a tube with a balloon into the narrowed area and inflate the balloon to compress plaque deposits against the side of the artery.

That is why heat can melt ice, vaporize water and cause bodies to expand.

We know that the universe is expanding and cooling, so earlier on it was smaller and hotter.