Turn off the flash (IMO you’ll be a better photographer if you never turn it back on).
Photo taken without flash:
Photo taken with flash:
Which one is best? 😉
When I’m absolutely desperate for light without a flash, I use this technique:
1. Choose ISO as high as it will go
2. Shoot RAW if possible
3. Use aperature-priority with the lowest f-stop on the fastest lens I have (f1.8 or lower if you can).