What is that big light in the middle of the milky way?

It is the galactic center. This is where stars are most concentrated and it appears brighter than the outer edges. The galaxy itself resembles the planet Saturn, with a bulge in the center of a large disk shape. The solar system is part of the outer disk. When we look into it, we see a faint band of light stretching across the sky. I suppose early astronomers thought it looked like a streak of milk. If the solar system were in the galactic center, there would be so many stars in the sky the night would resemble the day.