Distant, ancient galaxies are now giving scientists more hints that a mysterious power which is called dark energy it may not be what they thought.

Astronomers know that the universe is moving away at an accelerating rate and have wondered for decades about what it could possibly speed up. They think a strong, steady force is at play, one that fits well with the main mathematical model that describes how the universe behaves. But they can’t see it and don’t know where it comes from, so they call it dark energy.

It’s so vast that it’s thought to make up almost 70% of the universe — while ordinary matter like all the stars and planets and humans make up just 5%.

But the findings published earlier this year by an international research collaboration of more than 900 scientists from around the world caused quite a surprise. As scientists analyzed how galaxies move, they found that the force that pushes or pulls them doesn’t seem to be constant. And the same team published a new, broader set of analyzes on Tuesday that provided a similar answer.

“I didn’t think such an effect would happen in my lifetime,” said Mustapha Ishak-Boushaki, a cosmologist at the University of Texas at Dallas who is part of the collaboration.

It’s called Dark Energy Spectroscopic Instrument and is using a telescope based in Tucson, Arizona, to create a three-dimensional map of the universe’s 11-billion-year history to see how galaxies have clustered over time and space. This gives scientists information about how the universe evolved and where it might be headed.

The map they construct would make no sense if dark energy were a fixed force, as it is believed to be. Instead, the energy seems to change or fade over time. If this is indeed the case, it would overturn astronomers’ standard cosmological model. It could also mean that dark energy is very different from what scientists have previously thought – or that something else might be going on.

“It’s a time of great excitement, but also confusion,” said Bhuvnesh Jain, a cosmologist at the University of Pennsylvania who is not involved in the research.

The latest finding points to a possible explanation from an older theory: that over billions of years of cosmic history, the universe expanded and galaxies clustered as Einstein’s general relativity predicted.

Astronomers say they need more data to disprove a theory. They hope that observations from other telescopes and new analyzes of the new data in the coming years will determine whether or not the current view of dark energy is correct.