Answer:
A force exists that we knew nothing about that causes the acceleration.
Explanation:
In 1998, scientists observed from distances of supernovae in remote galaxies that the enlargement of the universe is accelerating. Besides, there is evidence that suggests that the universe is flat and infinite, as well as accelerating in its expansion. In that respect, the force suspected to be responsible for this is called dark energy.
Real gross domestic product (GDP) is an inflation-adjusted measure that reflects the value of all goods and services produced by an economy in a given year, expressed in base-year prices, and is often referred to as "constant-price," "inflation-corrected" GDP or "constant dollar GDP."
https://www.investopedia.com/terms/r/realgdp.asp
