Cosmic Egg

Now we know which came first. (But then, who laid it?)

"It is said to have emerged from an egg in a place where there was nothing, then shaped the world."

—Pokemon Platinum's Pokédex description for Arceus

A common Creation Myth Motif: the world, God, or some primordial entity comes into existence by hatching from an egg or similar structure.

This is generally used so that the universe's life-cycle mimics those of many of its creatures, giving the trope metaphorical significance as a symbol of life's beginning. As such, it is found in many religious stories and ancient mythologies, but it is also commonly found in many modern Mythopoeias as a way to begin the narrative.

See also Primordial Chaos, which may be where these are located, and World Tree, which has similar connotations.

Examples of Cosmic Egg include:

Anime and Manga

This is said to be the theory behind Reality Marbles in the Nasuverse—the Mental World of the magus is what's inside the Cosmic Egg, and the Reality Marble is the ability to switch the world outside the egg with the world inside. This "creation" is only temporary though; since the event is an unnatural occurrence, the larger world will attempt to revert the disturbance back to normal, which is why Reality Marbles require a large amount of prana to maintain even for a few minutes.

Comic Books

According to one story, this is how Galactus managed to survive the last big crunch.
This is how worlds are born in the Books of Magic. Tim restores the dying realm of Faerie by finding a world egg to hatch a new Faerie.
Buffy the Vampire Slayer season 8 shows the Seed of Wonder, which has the shape of a red egg about the size of a football. According to Spike, "The world came from the Seed."

Fan Works

At the end of the second story in the Harry Potter/Sword Art Online crossover series Mystic Knight Online by jgkitarel, Mystic Knight Online: All The World's Made Strange, the protagonists receive "The World Seed" -- essentially a semi-automated framework for generating virtual worlds -- from CARDINAL, one of the AIs Akihito Kayaba created to oversee the original Aincrad setting/server. In virtual space it looks like a glowing egg with lines of data faintly visible scrolling across it.

Literature

In The Gods Themselves by Isaac Asimov, these are used as fuel.
Discworld:
- One of the many origins of the Discworld universe given by the Great Spells in The Light Fantastic. Apparently, the Cosmic Egg was "distinctly rubbery."
- And in the novel Eric, it turns out that life was created by Rincewind (who had gone back in time) tossing away the Egg-and-Cress sandwich given to him by The Creator.
Appears in Tau Zero at the end of the universe, during the Big Crunch.

Music

Provided the name and album cover for Wolfmother's second full album.

Oral Tradition, Folklore, Myths and Legends

Older Than Dirt: The Rig Veda (and later, some Puranas) use this to describe the universe in its early stages in Hindu Mythology.
The Orphic Egg of Pelasgian creation myth which predates Classical Mythology and even the Greek language.
The Taoist idea of Pangu from Chinese Mythology is this trope.
Mentioned in the Kalevala.
Egyptian myth.
- And given that much of Judaism (and Christianity) developed because of contact with Egypt during its period of bondage, it's not surprising that the opening line "God created the heaven and the earth", the word "created" is supposedly similar to that for giving birth.
In Hawaiian mythology, the fire-goddess Pele carries an egg from her original home in Tahiti to the Hawaiian archipelago. The egg hatches out her fully-formed sister Hi'iaka, the goddess of dance. (In fact, Hi'iaka's much longer full name translates (very) roughly as "carried in the arms of Pele").

Video Games

Rosalina's starship, in Super Mario Galaxy has a warp field that makes it look like an egg. Given that she's apparently the overseer of the rebirth of the universe, this is likely intentional.
Arceus, the Creator God of Pokémon; its PokeDex entry in Pokémon Black and White claims, "It is said to have emerged from an egg in a place where there was nothing, then shaped the world."
While it's never stated whether or not the universe itself hatched from them, the Cosmic Eggs of Shin Megami Tensei: Strange Journey have the power to remake the Earth as a world of absolute Law or Chaos. They can also be detonated in order to destroy the Schwarzwelt instead.
- By gum, Tokyo in Shin Megami Tensei Nocturne sure is round like the inside of a cosmic egg! Which it is, and it's up to the main character to determine what kind of world will emerge from it. Or he can destroy the restarter mechanism and goes on a Rage Against the Heavens.
In Oracle of Tao, Ambrosia uses one of these to make all existence real (before that, it was more a Lotus Eater Machine with herself as the only real person).
The Precursor stone in Jak II Renegade is the last Precursor egg.

Real Life

Resurrected as an idea in modern science with the Big Bang: for the first few Planck Times the universe was a highly-compressed ball of proto-universal stuff. More often thought of as this within the big bang/big crunch model of an oscillating universe (now discredited by discovery of dark energy).