In 2004, Dr. Simon Singh wrote a book entitled, Big Bang: The Most Important Scientific Discovery of All Time and Why You Need to Know About It. On page 265 of that book he attributes a quote to Dr. Lev Landau, a Nobel Laureate in physics. According to Singh, Landau said:
Cosmologists are often in error, but never in doubt.
Regardless of whether or not Dr. Landau actually said this, it is an insightful statement. Most cosmologists have absolutely no doubt that the Big Bang Model is an accurate description of the history of our universe. When that model seems to contradict observational data, rather than doubting the model, they add something to it in order to force it to be in compliance with the data.
For example, in 1998, some observational data indicated that the expansion of the universe is accelerating. This didn’t agree with the current interpretation of the Big Bang Model, which suggested that the rate of universal expansion should be decreasing, since gravity should be attracting all sources of mass to one another. As more and more data supported the acceleration, cosmologists started to rely on dark energy, a mysterious form of energy that counteracts the effects of gravity.
In the currently-accepted form of the Big Bang Model, just under 70% of all the energy of the universe is dark energy. Cosmologists don’t know anything about it, but most of them have no doubt that it exists, because it forces the Big Bang Model into compliance with observations. Since they have no doubts about the Big Bang Model, they have no doubts about the existence of dark energy. It’s just that simple.