From Halopedia, the Halo wiki
The following are a list of quotes said by Cortana in the Halo series.
Halo: The Fall of Reach
"This Galaxy is vast; its wonders and beauty are almost unfathomable. But the galaxy also hides dark secrets, some of which have lain dormant since the beginning of time itself. There is a danger in secrets, both in seeking and in knowing. Some things are meant to be hidden from view. Some mysteries defy understanding, and sometimes even the things we think we know are untrue. Some secrets should remain untouched. The Halos were one such secret."