Here's why:

* Dark Matter: Scientists believe that about 85% of the Milky Way's mass is made up of dark matter. This mysterious substance doesn't interact with light, making it incredibly difficult to detect directly.

* Visible Matter: The remaining 15% is made up of the stuff we can see, like stars, gas, dust, and planets.

* The Missing Mass: The 70% figure represents the amount of dark matter that we're still trying to understand and account for.

While we've made significant strides in understanding the Milky Way, the vast majority of its mass remains a mystery.