Eldar were the first race in the stars, the first to decode the Runes and markings that made a helm. The first ship , a massive derelict ship crashed on their planet 100000 years ago. The site now is a holy place for any eldar.
The eldar formed the navigators guild, which grew in power and at one time ruled the eldar empire. As more races entered space, the more the navigators guild became its own entity outside the eldar culture. The eldar still reference themselves as the firstborn of space, but have acknowledged that they have lost control of the navigators guild.