The light from your flashlight does travel in a straight line until it hits an object or is scattered by particles in the atmosphere. The reason it doesn't continue infinitely into space is because the Earth's atmosphere acts as a barrier.
When light travels through the atmosphere, it interacts with various particles and molecules present in the air. These interactions can cause the light to scatter or be absorbed, which reduces its intensity as it travels through the atmosphere. The Earth's atmosphere is primarily composed of gases, such as nitrogen and oxygen, as well as other trace elements and particles like water vapor and dust.
Additionally, the atmosphere is not completely transparent to all wavelengths of light. Certain wavelengths, such as ultraviolet (UV) and some infrared (IR), are absorbed by the atmosphere, while others, like visible light, are less affected. This is why we can see sunlight during the day when the Sun's light passes through the atmosphere.
So, when you shine your flashlight, the light will travel until it encounters particles in the air or surfaces that scatter or absorb the light. It doesn't continue indefinitely into space because the atmosphere acts as a barrier, causing the light to disperse and eventually become too weak to detect.