Dark energy is one of the latest additions that scientists have added to their model of the universe to explain their observations. Most such things in science start out as an unproven part of a model, inserted as a leap of faith. Sometimes they catch on and are adopted by other scientists.
But dark energy never came to earth, took on human form, demonstrated incredible wisdom and power, suffered a horrible death, and then resurrected itself... just to get our attention.
That's the key missing link that unbelievers overlook. Start with those observations and derive everything else from them if you want a better model of the universe.