If you google for 'what is rate of expansion of universe' you get
Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 miles) per second per megaparsec (a megaparsec is roughly 3 million light-years).
Can anyone please explain what does the unit mean? what is 'per second per megaparsec'? I believe x km/s would suffic the rate of expansion. Not sure what 'per megaparsec' specifies?