Mea Lore

The burning heart of our Earth is of a heat and energy that mirrors the Sun. This burning heart is the sun of the Aerth, the inner-world. It is the Light of the Inner-world. It is big. It is great. It is Light. Therefore it was called the Lux.
All Light, logically, finds its source in the Lux. All the light, absence of dark, and the Light, absence of reality comes from there. All the magic that ever found its way to the surface began in the burning heart. Every magical creature that ever pestered a surface myth came from the burning heart. All the magical humans too for that matter.
Merlin? Dragons? Ogopogo? Mermaids? Bigfoot? Every last one had been banned to the surface world after their misdeeds inside the Aerth. Some learned their lessons, like the misunderstood Merlin. Some just got worse, like the Greek Gods. Either way the residents of Aerth weren't bothered by them anymore.
Well then. You may ask how this magic, or Light, found its way from the Lux into the residents of the Aerth. How did the Light separate from that brilliant globe and be consumed by the creatures?
That happened way back. Such a long, long way back that no one was around to witness it. All that can be surmised was that there was a cataclysmic destruction of the surface world. Some say it was a comet. For our purposes we shall agree with them. This comet wiped out everything above and would have entirely smashed the whole globe, splitting it like a watermelon, if it hadn't been for the Lux. This Lux kept the outer shell from cracking open entirely but the stress of the impact also cracked the Lux. Now, on a cloudy day one can look directly at the Lux and see striations marbling its surface. At night, in the cool, these weakened points break off. In fact, since light is taken from the burning orb in this process, that is what brings the night into the Aerth.
The night sky was a magnificent one and the dancing lights inside it always changed. Some nights the sky seemed a solid white, other's barely had as many dots as your hand has fingers. It would be too much to ask the wee creatures dotting the shell of the Aerth to know what was happening on the other side of their globe. So it was a long time before anyone discovered that when a bunch of people died, life leaving their shell of a body, the night sky was the brightest it had been. 
Theories have flourished about this fact. Perhaps the Lux was crying, like the rest of the town that night. Perhaps the Lux is sentient and was trying to make everyone feel better not closing extreme darkness upon their sorrow. Perhaps the Lux was indeed sentient and also mean, laughing out more light when it saw humans mourning something that never affected itself. 
Whatever the theory, the true reason was that the Lux drew the life/light of a person unto itself. All those persons were swallowed by the life, the heart of the world. These bits of life made the Lux brighter. When the beings in the Aerth began to chart the night sky and the circumstances of each day, their minds drew a connection and the above theory was born. These gobs and bits of light gained the name Mea (pronounced May-ah). 
Thus the Mea was born.
Meas bob about the night sky like drunken stars. They are hardly constant enough to sail by, however sailors on distant seas always knew how to find a Magna by them. Meas are drawn to Magnas in fact. Thus comes the phrase, "Follow the Meas Find the Land," and, "A Mea at Sea finds the Magna at land," and sailors had a few years smooth sailing following these. It held true. Held true until the Aquae Magna took her home under the sea. Then the Meas tried to draw close to her by dancing directly over the ocean itself. Land was nowhere to be seen. The Sailors, low on food and fresh water, cursed fluently when they had followed the Meas in the hopes of a bit of land. The phrase changed to "Never trust a Mea."
Through the Light of the Meas, the Magnas are chosen. When the Magna gets their hands on the Mea light they have the ability to manipulate the world. The inner-crust has been altered several times because of Magna Wars and sometimes by the creatures they created. Aerth is no longer a perfect hollow sphere and the boiling seas and launching mountains took their toll even through the crust onto the surface world. This, however, is not the place to go into that history.
This is the place to discuss Meas and their delightful impact.

Geography

Mea substance is often debated. Some believe the substance to be negligible, the balls being solely made of Light and therefore not able to be touched. This theory is disproved by the Magnas' obvious manipulation and consumption of them. Others believe Mea have a slight slip to their surface, like a pat of melting butter, which accounts for the jaunty way they slice and bob through the night sky. However, this subject has never been cleared up.
The Magnas themselves have differing opinions about the precise texture of a Mea. Considering some of the Magnas themselves aren't consistently of a solid form, this is to be expected. How does the wind feel a Mea? How do they feel to the sea? Etc.

The purpose of Meas is also debated by the curious-minded, those who refuse to call a question answered even after it has been answered. Some believe the clear answer is that Meas are purposeless.
However, the simple truth is, besides the inconsistent light and Magna's empowerment, the Meas create the night and day of the Aerth.
At the end of each night the recalcitrant Meas bob back home to their cracks in the Lux. It's brightness increases. Thus comes the Day.

The real question is not, "What are they made of?" nor "Why do they exist?"
But "Why do they return?" and "Who calls them home?"