How great must the force of gravity be to bend light in space? For example, does light crossing near the centre of our galaxy get refracted? Or would it require a much stronger force?
Any gravity bends light.
With very sensitive instruments, we have measured the deflection of light caused by our own sun.
We first became able to measure the deflection of light caused by our sun's gravity reliably in the 1960's. Some attempts and claims had been made before that, but the experimental error was quite large by comparison with the tiny effect being measured.
It is somewhat easier to see the deflection of light caused by white dwarf stars.
Being similar in mass to our sun, but about the diameter of Earth, a typical white dwarf has a surface gravity about
The Kepler space telescope has observed microlensing caused by white dwarf stars in binary systems with red giants.