The experiment went awry. The world is gone... sealed away or lost to the void. You just managed to activate a failsafe in time, saving but the smallest portion of the world. But there is still hope - perhaps the world can still be repaired - chunk by chunk.

Chunk By Chunk is a mod that starts the world with just a single chunk (16x16 column of world), but provides the ability to grow the world, adding additional chunks over time.

Blocks:

World Forge: Can crystalize stone and dirt blocks into world crystals and ultimately into World Cores, used to craft Chunk Spawners. An initial chunk spawner is found hidden in a chest at the bottom of the first chunk.
Chunk Spawner: Spawns a new chunk, as the world intended.
Unstable Chunk Spawner: Spawns a new chunk. This chunk can be from anywhere in the world.
Biome themed chunk spawners: A variety of these exists. These spawn new chunks, but altered to fit within a theme. Note that ocean chunks will still be filled with water.
World Scanner: Can map out the unspawned chunks for the presence of a provided block or liquid. Powered by world crystals or cores.
World Mender: Will automatically spawn chunk outwards from the mender, given cores or spawners.

There are two options for how the world is generated - missing chunks are either empty (making a skyworld), or are replaced with bedrock (sealing them). The default is sealed, mostly for less performance impact.

You can configure Chunk By Chunk from the main menu, using the Mods... menu prior to creating a world to configure the settings for that world. For fabric or quilt this requires the Mod Menu mod.