Dark Energy: The Mysterious Force Accelerating Universal Expansion

Sixty-eight percent of the universe is made of something we cannot see, touch, or directly detect. Dark energy is the mysterious force accelerating the expansion of the universe, pushing galaxies apart from each other at ever-increasing speeds. And now, new data suggest this force may be changing over time.

📖 Read more: Dark Matter: The Invisible Force Shaping the Universe

🌌 What Is Dark Energy

Dark energy makes up approximately 68% of the total energy content of the universe. It is not a form of radiation or particle — it is a property of space itself that causes repulsive gravity. While ordinary gravity pulls objects together, dark energy pushes them apart. And it is winning the battle — the universe is expanding faster and faster.

To understand the scale: of the total energy-matter content of the universe, just 5% is ordinary matter (atoms, stars, planets), 27% is dark matter, and the remaining 68% is dark energy. Essentially, 95% of the universe remains a mystery.

💡 The Discovery

Dark energy was discovered in 1998 by two independent teams of astronomers studying Type Ia supernovae. Saul Perlmutter, Brian Schmidt, and Adam Riess expected to find that the universe's expansion was slowing down. Instead, they discovered it was accelerating — a finding that earned them the 2011 Nobel Prize in Physics.

Ironically, Albert Einstein had predicted something similar in 1917 with the “cosmological constant” (Lambda, Λ). He later disowned it as his “greatest blunder.” It turned out he was right all along.

🔬 DESI — Is Dark Energy Changing?

In 2024, the DESI (Dark Energy Spectroscopic Instrument) published results that stunned the scientific community. By analyzing the light of millions of galaxies, DESI found that dark energy may not be constant — but changing over time.

This concerns the w parameter, which describes the pressure-density relationship of dark energy. If w = −1, dark energy is constant (as predicted by the Λ-CDM model). But DESI data hint that w is changing, something that could overturn fundamental assumptions of modern cosmology.

🔭 Euclid and Rubin

To definitively answer this question, two colossal programs are underway. ESA's Euclid telescope launched in July 2023 with a mission to map the geometry of the universe to a depth of 10 billion light-years, revealing how dark energy shapes the large-scale structure.

In parallel, the Vera C. Rubin Observatory (formerly LSST) will begin observations that will survey the sky with unprecedented accuracy, detecting billions of galaxies and measuring dark energy's impact on the structure of the universe. Together, Euclid and Rubin will provide the most accurate data we have ever had.

☠️ The End of the Universe

Dark energy is not merely an academic question — it determines the ultimate fate of the universe:

• Big Freeze: If dark energy remains constant, the universe will expand forever, cooling gradually until absolute silence.
• Big Rip: If dark energy strengthens (phantom energy, w < −1), it will eventually tear apart galaxies, atoms, and even spacetime itself.
• Big Crunch: If dark energy weakens or reverses, the universe will collapse back into a singularity.

🤔 What We Don't Know

Dark energy remains the greatest mystery in modern physics. Quantum mechanics predicts that empty space should have enormous energy, but the observed value is 10¹²⁰ times smaller than the prediction. This discrepancy — known as “the cosmological constant problem” — is considered the worst prediction in the history of physics.

For now, we do not know what dark energy is, why it exists, or whether it is changing. What we do know is that it controls the fate of the universe. And for the first time in history, we have the tools to uncover it.