In the 1970s, astrophysicists studying the motion of nearby galaxies noticed something strange: our entire Local Group, the Milky Way included, was being pulled off its expected cosmic path at over a million miles per hour, toward something no telescope could actually see. The pull was coming from directly behind our own galaxy's crowded disk of stars and dust, in a blind spot so complete that more than fifty years later, no single telescope has ever produced a full, unobstructed picture of whatever is causing it.
Background: A Pull With No Visible Source
The discovery came from measuring what astronomers call peculiar velocities, the small deviations in a galaxy's motion left over after accounting for the universe's overall expansion. In a landmark 1988 study, astronomer Donald Lynden-Bell and colleagues used distance measurements to 400 elliptical galaxies to show that a huge swath of the local universe was flowing consistently toward a single point, a flow too coordinated to be random. They coined the term "Great Attractor" for whatever unseen mass concentration was responsible.
The problem was where that point turned out to be. The Great Attractor sits almost directly behind the Milky Way's own galactic plane, inside a roughly 20-degree band of sky known to astronomers as the Zone of Avoidance, where our own galaxy's dust, gas, and dense star fields block most direct observation. Astronomers had identified a source powerful enough to redirect the motion of our entire galactic neighborhood, and could barely see it at all.
Peering Through the Dust
Studying something hidden behind an opaque foreground has forced astronomers to rely on wavelengths of light that can penetrate dust rather than being blocked by it. Infrared observations by the European Space Agency's Infrared Space Observatory in the late 1990s provided some of the first real clues. Radio telescopes, detecting neutral hydrogen gas that radio waves can pass through far more easily than visible light, mapped large-scale structures crossing the galactic plane through dedicated surveys like HIZOA. X-ray telescopes, sensitive to the superheated gas trapped inside massive galaxy clusters, added yet another layer, eventually helping identify ACO 3627, also known as the Norma Cluster, as a leading candidate for the Great Attractor's core. A 2017 spectroscopic study found further evidence of a large cluster or supercluster hidden in the constellation Vela, and a dedicated X-ray survey called CIZA has since catalogued roughly 150 separate galaxy clusters in the region, suggesting the Great Attractor isn't one single object at all, but a sprawling, extended concentration of many.
A Mystery That Kept Getting Bigger
In 2014, astronomer Brent Tully and colleagues used these accumulating maps to identify something even larger: Laniakea, Hawaiian for "immense heaven," a supercluster containing an estimated 100,000 to 150,000 galaxies, including our own Milky Way, with the Great Attractor sitting at its gravitational center. For a while, that seemed to close the picture. Then newer data complicated it further. Laniakea itself, along with several other neighboring superclusters, appears to be flowing toward something still larger: the Shapley Concentration, first noted by astronomer Harlow Shapley in the 1930s and now estimated to potentially span up to ten times Laniakea's own volume. As one researcher involved in recent Shapley Concentration mapping put it, current surveys may simply not yet be large enough to capture the full extent of the structures pulling on our galaxy.
Cosmic Anomalies at a Glance
| Anomaly | What's Observed | Leading Explanation | Status |
|---|---|---|---|
| Fast Radio Bursts | Millisecond-long, extremely energetic radio pulses from deep space | Magnetar activity, confirmed for at least one source | Active research; likely multiple distinct causes |
| Planet Nine | Unusual clustering in the orbits of distant icy solar system objects | An undiscovered large planet in the outer solar system | Unconfirmed; new survey telescope now searching |
| The Great Attractor | Thousands of galaxies, including our own, drawn toward one region | A massive concentration of galaxy clusters, hidden behind the Milky Way | Partially mapped; part of an even larger structure |
| CMB Cold Spot | An unusually cold, large region in the cosmic microwave background | Statistical fluke, or a massive cosmic void | Debated; no fully settled explanation |
| Axis of Evil (CMB Anomaly) | Unexpected large-scale alignment in cosmic background radiation patterns | Possible instrument or foreground contamination | Unresolved; challenges assumptions about cosmic randomness |
Theories and Explanations
An ordinary, if enormous, concentration of galaxy clusters. The dominant explanation is refreshingly unexotic: the Great Attractor's pull is caused by ordinary matter, galaxies, hot gas, and dark matter, concentrated in a region containing dozens of galaxy clusters, estimated at somewhere between 10^16 and 10^17 solar masses. What makes it mysterious isn't the physics; it's simply that our own galaxy happens to sit directly in the way of seeing it clearly.
An extended structure rather than a single point. The CIZA X-ray survey's discovery of roughly 150 distinct clusters in the region supports treating the Great Attractor as a broad, complex structure rather than one discrete object, a more extended and diffuse mass concentration than the original 1980s studies could have detected.
Part of a larger flow toward Shapley. Researchers have found no strong evidence of galaxies falling into the far side of the Great Attractor the way you'd expect if it were the true endpoint of the flow, a clue supporting the idea that the Great Attractor is really just a way station within a much larger flow toward the more distant Shapley Concentration.
A genuinely incomplete map. The most heavily obscured slice of the Zone of Avoidance, within about one degree of the galactic plane itself, remains largely unsurveyed even with modern instruments, meaning some portion of this mystery may stay observationally out of reach for the foreseeable future regardless of which wavelength astronomers use.
The Curious Connection
The Great Attractor uses the same basic method as this series' previous entry on Planet Nine, inferring an unseen mass from its gravitational effect on everything around it, but at a scale roughly a trillion times larger. Where Planet Nine's mystery could plausibly end with a single confirmed direct detection from a telescope like the Vera C. Rubin Observatory, the Great Attractor's mystery turned out to be recursive: mapping it in enough detail to identify the Norma Cluster and the broader Laniakea Supercluster didn't close the case, it revealed an even larger structure, the Shapley Concentration, quietly pulling on the very supercluster the Great Attractor sits inside of.
That nested quality, one solved layer of mystery simply revealing a bigger one underneath, is a genuinely different flavor of "unsolved" than anything else in this series so far. It's a fitting bridge to this series' next entry, which turns to an anomaly at the largest observable scale that exists at all: an unexplained cold spot embedded in the faint afterglow of the Big Bang itself.
Frequently Asked Questions
What is the Great Attractor?
The Great Attractor is a massive concentration of galaxy clusters whose gravity is pulling our Local Group of galaxies, including the Milky Way, off its expected cosmic path, first identified through galaxy motion studies in the 1970s and 1980s.
Why can't astronomers see the Great Attractor directly?
It lies almost directly behind the Milky Way's own galactic plane, within a region called the Zone of Avoidance, where our galaxy's dust, gas, and dense star fields block most visible-light observation.
What is the Great Attractor actually made of?
Based on infrared, radio, and X-ray surveys, it appears to consist of an extended concentration of ordinary galaxy clusters, including the Norma Cluster, along with roughly 150 other identified clusters in the surrounding region.
Is the Great Attractor the largest structure pulling on our galaxy?
No. Research suggests the Great Attractor sits at the center of the larger Laniakea Supercluster, which itself appears to be flowing toward an even more massive structure known as the Shapley Concentration.
Will the Great Attractor ever be fully mapped?
Possibly not completely. The most obscured portion of the Zone of Avoidance remains largely unsurveyed even with modern telescopes, though ongoing infrared, radio, and X-ray surveys continue to fill in more of the picture.