Navigating a Universe of Unknowns

Written by Isla Madden

Ancient Greek philosopher Democritus proposed the existence of indivisible particles—the term “atom” derives from the Greek word atomos, meaning “uncuttable.” Despite this so-called indivisible atom ultimately being split by Meitner, Hahn, and Strassmann in 1938, Democritus’ early concept laid the philosophical groundwork for atomic theory. Aristotle attempted to categorise the physical world by proposing four classical elements: earth, water, air, and fire. While primarily speculative, these early efforts to understand the world laid the foundation for centuries of scientific inquiry.

At the dawn of the Universe, matter existed as a quark–gluon plasma, a primordial state from which all atomic structures ultimately formed. Anything that has mass and occupies space constitutes matter. The word baryon comes from the Greek barys, meaning “heavy.” Baryonic matter is built from quarks, elementary particles that combine to form protons and neutrons. Quarks are held together by the strong nuclear force, carried by gluons, which confines them within larger particles. Baryonic matter makes up the world we interact with—you, the Earth beneath your feet, and the heavens above—yet it contributes less than 5% of the Universe’s total mass–energy density.

Approximately 27% of the Universe’s mass–energy does not reflect, emit, or absorb light—this is dark matter, which interacts with ordinary matter through gravity. Observations of galaxies and their stars revealed velocities higher than their visible matter could account for, with stars in spiral galaxies orbiting at nearly constant speeds, contrary to predictions based on Newtonian physics. These unexpected motions suggest the presence of a “missing mass” influencing the dynamics of cosmic structures, though its exact nature remains unknown. 

The remaining majority of the Universe’s mass–energy is dominated by dark energy, a mysterious force causing cosmic expansion to accelerate. Einstein introduced the cosmological constant (Λ) to preserve a static Universe, but later abandoned it after Hubble observed a linear relationship between a galaxy’s distance and its recession velocity—one of the first observational confirmations of an expanding Universe. Decades later, supernova observations showed that this expansion was accelerating, resurrecting Einstein’s Λ. The elusiveness of dark energy underscores that the pursuit of knowledge is propelled as much by the unknown as by the known.

In his dialogue Timaeus, Plato describes the Universe as a living being endowed with a soul, with humans as part of its order. The ancients recognised the cosmos not as a collection of isolated parts, but as a unified, dynamic whole. We are only capable of perceiving a small fraction of our cosmos. Dark energy remains elusive, and the ultimate fate of the Universe—whether endless expansion, steady state, or transformation—remains unknown, reflecting both the limits of observation and the ongoing pursuit of knowledge. Scientific progress is rarely linear—our understanding evolves through correction, refinement, and discovery.