2025 is turning out to be a busy year for astronomers – experts have made many discoveries and received a lot of new information about the universe. Space portal.com speak about the most important astronomical discoveries of the year.
New interstellar comet
The most notable event in the second half of 2025 is undoubtedly comet 3I/ATLAS, the third interstellar object found by astronomers in the entire history of space observations. The Chilean component of the ATLAS system discovered the comet on July 1, and by September the object had set above the Sun, making observation from Earth impossible. NASA and ESA had to use their fleets of spacecraft to continue studying the comet.
Scientists have discovered a few things. 3I/ATLAS is a comet, not a spacecraft, and has all the characteristics of a comet. Its chemical composition is similar to other comets in the solar system, which is a major discovery in itself. But there are certain differences. Therefore, 3I/ATLAS has a slightly higher carbon dioxide/water ratio than other comets, and it contains more nickel than iron.
The birth of a supermassive black hole
When the James Webb Space Telescope began taking deep images of space in 2022, it quickly began finding “little red dots” in the background. Astronomers do not know what they represent. Scientists initially thought the dots might be dwarf galaxies or dense star clusters in the early universe, but they are so bright that the standard cosmological model simply cannot explain their formation.
However, the spectrum of these small spots is not the same as that of a star. Finally, in September, astronomers proposed an answer: the dots are nothing more than supermassive black holes born inside giant, dense clouds of gas a billion years after the Big Bang. These holes can form by the direct gravitational collapse of a gas cloud or as a result of the merger of countless smaller black holes caused by the collapse of the cores of massive stars hidden in the cloud.
Dark energy weakens
The first batch of data collected by the DESI instrument revealed shocking news: dark energy, which drives the expansion of the universe, appears to be weakening. For context, this directly contradicts the leading hypothesis, which holds that dark energy is a cosmological constant and therefore never changes. Although scientists are not 100% sure about the new information, it is at least intriguing.
The year of biosignatures
In 2025, scientists discovered the most intriguing and controversial signs that we may not be alone in the universe.
Thus, the best evidence of the existence of life on Mars appeared in September thanks to the Perseverance rover – the device found pale red spots surrounded by dark matter. These “leopard spots” are not too rare on Earth, and they usually form in one of two ways: by exposure to hot, acidic environments or by biological factors. The rover also detected organic molecules in sedimentary rocks containing clay within the rocks, although Perseverance was unable to identify these molecules. Therefore, this discovery may indicate that there was microbial life in Jezero Crater 3.5 billion years ago.
And another notable biological signature could potentially be found on exoplanet K2-18b. In 2023, astronomers recorded signs of the presence of dimethyl sulfide gas, as well as methane and oxygen. According to scientists, this suggests that K2-18b is a planet with an extremely deep ocean surrounded by a dense, hydrogen-rich atmosphere.
The uncertain future of the Milky Way and Andromeda
The Milky Way and Andromeda Galaxy may not collide for the next 10 billion years. A new study published in 2025 concluded that galaxies could collide with each other with a 50-50 probability.
Looking at the gravitational pull of the Large Magellanic Cloud pulling on the Milky Way and the gravitational pull of the Triangle Galaxy pulling on Andromeda, scientists were able to better calculate how close they would get to each other in a series of simulations. In total, the critical distance is 650,000 light years – if galaxies step outside this boundary, a collision is inevitable in the next 10 billion years.
The largest black hole in history
In 2025, scientists discovered perhaps the largest black hole in the entire history of observation. The hole has a mass 36 billion times greater than the Sun, located at the center of one of the largest galaxies in the universe – the Cosmic Horseshoe.
Theories have been put forward about other, larger black holes, but the authors of the latest scientific paper assert that the masses of these black holes have not been directly measured – meaning no one knows the exact number for sure. But the mass of the black hole in the Cosmic Horseshoe was measured more directly and accurately by tracking the motions of groups of stars affected by its gravity.
