6 strange facts about the observable universe and why most of it is hidden

The universe we see through telescopes is only a fragment of reality. Beyond the dazzling galaxies and cosmic webs lies a hidden majority, dark matter, dark energy, and regions forever beyond our reach. From illusions created by warped spacetime to the prospect of an infinite cosmos, astronomers are uncovering truths that challenge our very sense of existence. These discoveries suggest that the observable universe is less a complete picture than a narrow window into something far stranger and vastly larger.

Gravity can bend light, and on cosmic scales this produces remarkable illusions. When light from distant galaxies passes massive foreground clusters, it is deflected, stretching and distorting into arcs, rings, or multiple images. These effects, known as gravitational lensing, include Einstein rings and the Einstein Cross. The James Webb Space Telescope has recently captured a striking example: a ‘question mark’-shaped distortion produced by a rare hyperbolic umbilic lens, the result of precisely aligned galaxies and clusters warping spacetime itself.

The observable universe contains an estimated two trillion galaxies, hosting about 10²⁴ stars, a septillion, far more than the grains of sand on every beach on Earth. On an even smaller scale, it holds roughly 10⁸⁰ hydrogen atoms, a number known as the Eddington number. These staggering quantities remind us that what we see in the night sky is only a tiny fraction of the universe’s true scale.

Dark energy drives the accelerating expansion of the cosmos, pushing distant galaxies away faster with time. Many of these galaxies are already receding so rapidly that no signal sent today will ever reach them, making more than 90 per cent of the observable universe effectively inaccessible. Although they never literally disappear, they fade from any possible contact, leaving us on an island of visibility surrounded by a receding sea of galaxies.

Galaxies are not scattered randomly but trace out a vast cosmic web, with filaments, clusters, and superclusters separated by enormous voids. Observations suggest our own Milky Way may sit within an immense cosmic void nearly two billion light-years across. If true, this unusual position could subtly influence how astronomers measure cosmic expansion, raising questions about how local conditions shape our view of the wider universe.

The Cosmic Microwave Background (CMB), the faint afterglow of the Big Bang, provides a map of the infant universe. Yet it shows puzzling irregularities. The so-called CMB Cold Spot is a region larger and cooler than expected, spanning about five degrees across the sky. Some propose it is caused by a vast supervoid along our line of sight, but its existence continues to challenge standard cosmological models.

Our galaxy is not stationary. It drifts toward a massive region called the Great Attractor, 150 to 250 million light-years away, which exerts a gravitational pull on hundreds of thousands of galaxies. Hidden behind the dense plane of the Milky Way, it is difficult to study directly. Even more speculatively, some observations once suggested a ‘dark flow’ of galaxy clusters moving coherently toward a mysterious direction, perhaps influenced by structures beyond our observable universe, though later data cast doubt on the effect.