Hi there Robin and welcome
There can be a number of causes for this
If you're aperture is set high (inversely a low number, f/2.8, f/3.5) it'll let a lot of light in. If you want to keep this high so the model is in focus but the background is out of focus, you could increase you shutter speed. the faster the shutter speed (the time it is open) the less light that can get in.Also, make sure your iso isn't on auto if you're shooting in Manual, and set it to 100, the lowest setting.Additionally, you can underexposing it using the the AV button above the Q button on the back . Whilst holding this, move the wheel on the top of the camera left, you'll notice the meters going from 3---2---1---0---1---2---3 exposure bar at the bottom of your screen or in your eyepiece. Moving the small rectangle left on this bar will make the images darker, which you can see happen in real time if you shoot in live view.
Let me know how you go please
The monitor does not reflect the try picture via the telescope which is why you need to compensate.
You would have to avoid using the computer to get better results
with the computer and this combination you will not get around this Robin.
Its how the monitor displays the image.
If you would like any additional information or assistance, please do not hesitate to let me know.
Else if you can take a second to rate my service by clicking one of the stars at the top of the screen then submit, that would be greatly appreciated.