Theory: What is Goo

Abstract
Goo is the substance used in the graffiti from Super Mario Sunshine, the creation of the non-portrait ghosts from the first Luigi's Mansion that were created by Vincent Van Gore, Vim from Mario & Luigi: Partners in Time, Paint from Paper Mario: Color Splash and in the Paintings found in Super Mario 64 and Super Mario Odyssey.

Arguments

 * Argument 1: Graffiti: Goo is created by "Liquid Ghost Energy" and "Lunoman Greenie Coffee". It dissolves when it comes into contact with liquids such as water. Graffiti from Super Mario Sunshine shares this trait with Goo. Goo can be formed into shapes to create new living beings, another feature of the Graffiti. Graffiti comes from the Magic Paintbrush, which was invented by Professor E. Gadd, the man who first discovered Goo.


 * Argument 2: Van Gore's Creations: The Generic, Non-portrait Ghosts seen throughout the first Luigi's Mansion were created by the Ghost Painter Vincent Van Gore. He uses his paint to create "zee life [he] gives [his] creations". This is very similar to the that the Magic Paintbrush can create with Graffiti.


 * Argument 3: Vim: Vim is referred to as the life force of the Toads. It is a Green liquid, Just like goo and has Toad Spirits flowing through it, which lines up with goo being created using Liquid Ghost Energy.


 * Argument 4: Paint (Paper Mario: Color Splash): Paint acts as a sort of life force for living beings in the Mario Universe, including Toads, just like Vim. Paint can become its own living being capable of possessing others and altering their emotions, much like a ghost. Concept art for the Black Paint possessing Bowser even shows that the Paint has a very similar design to the “ghosts” created by Vincent Van Gore in Luigi’s Mansion, with yellow eyes, extended arms and a wavy “tail”. (Keep in mind this was still a Concept Design and didn’t make it into the final game.)


 * Argument 5: Paintings (SM64 & SMO): The paint used in Graffiti from Super Mario Sunshine can be used to create portals between two locations. The paintings of Super Mario 64 and Super Mario Odyssey also act as portals between two locations. They even have the same ripple-like effect. Ghosts can be trapped inside Paintings and some can even trap other items and beings inside paintings. The Dark-Light Device (which was also invented by E. Gadd) makes the “Liquid Ghost Energy” visible to the naked eye. It also sets beings/items trapped in Paintings free.


 * Argument 6: Polterpup [as pointed out by Black Lava Cheep Cheep and Marhiin]: In Luigi's Mansion: Dark Moon, when Luigi dies but has a gold bone, the Polterpup will lick him to revive him. Since Liquid Ghost energy is present in the Polterpup and Goo is established to be a life force in this theory from the evidence above, the Liquid Ghost Energy could revive Luigi.