To be clear, light years are a measure of distance, not time. You are “looking at the past” because it took 100 million Earth years for the light to travel all the way to the Earth.
This has already been answered, but if you meant “how many miles is that equal to,” a light-year is roughy 6 trillion miles (6,000,000,000,000), so 100 million light years would be about 6 quintillion miles (6,000,000,000,000,000,000).