What is the biggest thing in the world?

Defining "biggest" depends on what you're measuring. There's no single biggest thing. Here are a few contenders, depending on the criteria:

• By mass: The most massive object is likely a supermassive black hole. These reside at the centers of most galaxies, and the largest known, at the center of the galaxy Holmberg 15A, has an estimated mass of 40 billion times that of our Sun. It's not "big" in terms of physical size, but its mass dwarfs everything else.

• By physical size (volume): This is tricky. The largest known structure in the universe is a galaxy supercluster called the Hercules-Corona Borealis Great Wall, extending for billions of light-years. However, defining its exact edges is difficult, and even larger structures may exist that we haven't discovered yet. It's crucial to note that it's not a cohesive object, more of a vast concentration of galaxies.

• By surface area: Again, related to the above, the Hercules-Corona Borealis Great Wall or similar large-scale structures would likely claim this title, though precise measurement is impossible.

• By diameter: While supermassive black holes are incredibly massive, their size is surprisingly compact. Considering only observable objects, again large-scale structures like the Hercules-Corona Borealis Great Wall would be at the top.

In short, there's no single answer to "the biggest thing in the world." The answer depends heavily on how you define "biggest." The observable universe itself could also be considered a contender, though it's not an object in itself.