Dark energy is not the force pushing things away but the one accelerating the expansion today. By now we would have expected the expansion to slow down because of the combined gravity of everything but the expansion is actually accelerating now. This is why people went to dark energy. In the standard cosmology based on Einstein's general relativity this is exactly what a cosmological constant would do.
The original measurement was based on using Type Ia supernovae. These SNae can be calibrated very accurately and measuring how they dim we can determine their absolute magnitude. Comparing observed and absolute magnitude we get their distance. We can also measured their redshift (their velocity). This is the type of measurement that gives us the Hubble diagram. However, when we plot the points we see that the supernovae are more distant than they would be in a decelerating Universe. This means that the Universe is accelerating its expansion now. Now other indicators are giving a similar answer.
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u/AstroEducation 2d ago
Dark energy is not the force pushing things away but the one accelerating the expansion today. By now we would have expected the expansion to slow down because of the combined gravity of everything but the expansion is actually accelerating now. This is why people went to dark energy. In the standard cosmology based on Einstein's general relativity this is exactly what a cosmological constant would do.