Euclid Telescope & Vera Rubin Observatory: Mapping Dark Matter & Dark Energy

Most of the Universe Is Missing

Here is a fact that still stops me cold every time I think about it: everything we can see — every star, every galaxy, every planet, every nebula, all of the gas and dust and everything in between — makes up roughly 5 percent of the total mass and energy content of the universe. The remaining 95 percent is invisible. About 27 percent is dark matter — something with mass and gravitational pull that does not emit, absorb, or reflect light. The remaining 68 percent is dark energy — a mysterious force driving the accelerating expansion of the universe.We do not know what dark matter is. We do not know what dark energy is. But two of the most ambitious astronomical projects ever built are now operational and designed specifically to answer those questions. Their names: Euclid and the Vera C. Rubin Observatory.

What Is Dark Matter?

The evidence for dark matter is overwhelming and comes from multiple independent observations:

• Galaxy rotation curves: Stars in the outer regions of galaxies orbit far faster than they should based on the visible mass alone. Something unseen is providing additional gravitational pull.
• Galaxy cluster dynamics: Galaxies within clusters move too fast to be held together by the gravity of their visible matter. Without dark matter, clusters would fly apart.
• Gravitational lensing: The light of distant galaxies is bent and distorted as it passes through foreground galaxy clusters. The amount of bending reveals far more mass in the clusters than the visible galaxies and gas can account for.
• The cosmic microwave background: The pattern of temperature fluctuations in the CMB — the afterglow of the Big Bang — matches models that include dark matter perfectly and fails without it.

The leading candidate is some type of weakly interacting massive particle (WIMP) that interacts through gravity and possibly the weak nuclear force but not through electromagnetism, which is why it does not emit or absorb light. Despite decades of laboratory searches, no dark matter particle has been directly detected. The mystery remains open.

What Is Dark Energy?

In 1998, two independent research teams discovered that the expansion of the universe is not slowing down (as gravity should cause) but speeding up. This discovery, which earned Saul Perlmutter, Brian Schmidt, and Adam Riess the 2011 Nobel Prize in Physics, was based on observations of Type Ia supernovae as distance markers.Something is counteracting gravity on cosmic scales, pushing the universe apart faster and faster. We call this dark energy, but the name is essentially a placeholder. It could be a property of space itself (Einstein’s cosmological constant), it could be a dynamic energy field that changes over time (often called quintessence), or it could be a sign that our understanding of gravity on very large scales is incomplete.Distinguishing between these possibilities requires mapping the large-scale structure of the universe with extraordinary precision — which is exactly what Euclid and Vera Rubin are designed to do. The first-ever image of a black hole, captured by the Event Horizon Telescope collaboration in 2019 — the supermassive black hole at the center of galaxy M87. Dark matter and dark energy represent even deeper mysteries that new observatories are now tackling. Credit: EHT Collaboration, CC BY 4.0

The Euclid Space Telescope

Euclid is a European Space Agency (ESA) mission that launched on July 1, 2023, and began science operations later that year. It sits at the Sun-Earth L2 Lagrange point — the same orbital neighborhood as the James Webb Space Telescope — where it has an unobstructed view of deep space, shielded from Earth’s heat and light.

What Euclid Does

Euclid carries two instruments: a visible-light camera (VIS) with a massive 600-megapixel detector, and a near-infrared spectrometer and photometer (NISP). Over its six-year primary mission, it is surveying approximately one-third of the extragalactic sky, measuring the shapes, positions, and redshifts of billions of galaxies out to a distance of 10 billion light-years.The key science technique is weak gravitational lensing. Dark matter, though invisible, has mass, and mass bends light. By measuring the tiny, systematic distortions in the shapes of billions of background galaxies caused by foreground dark matter, Euclid can reconstruct a three-dimensional map of the dark matter distribution across the universe. Think of it like mapping an invisible landscape by studying how it distorts the light passing through it.Euclid also maps the distribution of galaxy clusters over cosmic time. Because dark energy affects how quickly clusters grow, charting their abundance at different distances (and therefore different epochs) constrains the properties of dark energy.

Early Results

Euclid’s first science images, released in late 2023, demonstrated the telescope’s extraordinary wide-field sharpness. A single Euclid frame covers an area of sky comparable to about two full Moons while resolving individual galaxies at the limit of Hubble’s depth. The mission has already produced the most detailed wide-field images of galaxy clusters ever taken from space, and the full survey data is expected to transform our understanding of cosmic structure.

The Vera C. Rubin Observatory

While Euclid surveys from space, the Vera C. Rubin Observatory tackles the same questions from the ground. Located atop Cerro Pachon in the Chilean Andes at 2,663 meters elevation, Rubin is unlike any telescope that has come before it.

The Hardware

Rubin’s primary mirror is 8.4 meters in diameter, but what makes it truly unique is its camera: the LSST Camera, the largest digital camera ever built. It has a 3.2-gigapixel sensor — so large that a single image covers an area of sky about 40 times the size of the full Moon. The telescope is designed to survey the entire visible Southern Hemisphere sky every three nights, taking roughly 1,000 images per night.

The Legacy Survey of Space and Time (LSST)

The observatory’s primary program is the 10-year Legacy Survey of Space and Time. Over a decade, the LSST will build up the deepest, widest, most complete map of the southern sky ever assembled, detecting roughly 20 billion galaxies and a similar number of stars. It will measure the shapes of billions of galaxies for weak lensing (like Euclid, but from the ground), track millions of transient events (supernovae, variable stars, asteroids) in near-real-time, and catalogue the solar system’s small body population with unprecedented completeness.The LSST will generate approximately 20 terabytes of data per night. Over ten years, the full dataset will exceed 60 petabytes. It is big data on a scale that astronomy has never dealt with.

Why It Is Named After Vera Rubin

Vera Rubin (1928-2016) was the American astronomer whose observations of galaxy rotation curves provided some of the most compelling evidence for dark matter. Her meticulous work in the 1970s and 1980s showed that stars at the edges of galaxies orbit at unexpectedly high speeds — a discrepancy that could only be explained by large amounts of unseen mass. Naming this observatory after her is a fitting tribute to a scientist who spent her career studying the invisible.

What Will We Learn?

Together, Euclid and Vera Rubin will address several of the biggest open questions in cosmology:

• Is dark energy constant or changing? If dark energy is Einstein’s cosmological constant, it has the same strength everywhere and at all times. If it varies, the expansion history of the universe will show telltale deviations from the constant model. Both missions are designed to detect these deviations at the percent level.
• What is the total mass of neutrinos? Neutrinos have mass, but we do not know exactly how much. The large-scale distribution of matter is sensitive to neutrino mass, and precision lensing surveys can constrain it from above — a cosmological measurement of a particle physics property.
• Is general relativity correct on the largest scales? Modified gravity theories have been proposed as alternatives to dark energy. By comparing the growth of structure (measured by lensing) with the expansion rate (measured by supernovae and galaxy clustering), Euclid and Rubin can test whether gravity behaves as Einstein predicted at cosmic distances.
• What is the three-dimensional structure of the cosmic web? The universe’s matter is arranged in a vast network of filaments, walls, and voids — the cosmic web. Mapping this web in detail tells us about the initial conditions of the universe and the physics that shaped its evolution.

Galaxies scattered across a deep Hubble field. Euclid and Vera Rubin will catalog billions of galaxies to trace the distribution of dark matter and the effects of dark energy across the observable universe. Credit: NASA/ESA/Hubble

What This Means for the Rest of Us

You do not need to be a cosmologist to appreciate what is happening. Euclid and Vera Rubin represent humanity’s most ambitious attempt to understand the fundamental nature of the universe. The data they produce will be publicly available — Rubin’s data releases, in particular, are designed to be accessible to the broader scientific community and eventually the public. Citizen science projects will almost certainly emerge to help classify the billions of objects these surveys detect.And for amateur astronomers, these missions add a layer of meaning to every observation. That faint galaxy smudge you captured through your backyard telescope is part of the same cosmic web that Euclid and Rubin are mapping. The photons hitting your sensor have traveled through dark matter halos, been subtly deflected by invisible mass, and arrived carrying information about the large-scale structure of the universe. The difference between professional cosmology and backyard astrophotography is a matter of scale, not of kind.We are living through the decade when the invisible universe starts to come into focus.