Which wire is neutral and hot?
You've got hot and switched hot on the existing switch. If the box is grounded properly then you will get 120V from hot (or switched hot when it is switched on) to the box. You will also get 120V from hot to neutral when the switch is off. If I understand it correctly (and Harper or one of the other real experts will correct me if I'm wrong), the circuit is actually:

hot -&gt; switch -&gt; switched hot -&gt; light fixture -&gt; neutral


The problem is often, particularly in older installations, that neutral is not present within the box because it is not needed by older switches. But neutral is needed by the light - just often that neutral will bypass the actual box since it wasn't (without a smart switch and prior to current code) required inside the box for any reason.

In your case, if I understand the pictures correctly, multiple black (hot) and white (neutral) wires nutted together. That indicates you have some additional circuits - e.g., outlets or more lights controlled by another switch - that are sharing the hot &amp; neutral with this light switch &amp; fixture. That is perfectly normal, but the result is that the neutral for the light fixture depends on those white wires being together. Removing the wire nut breaks the light circuit, and probably some other outlets or lights elsewhere in your house.

Bottom line: The white wires nutted together should be neutral, and you should be able to add another wire (pigtail) from the smart switch into that bundle/wire nut.

One more thing to be watch for: Hot vs. Switched Hot

In an ordinary single (i.e., one switch, as opposed to 2 switches - which is actually called 3-way) switch, it does not matter in any practical way which wire (hot vs. switched hot) is connected to which screw. However, with a smart switch you must make sure that hot and switched hot are connected according to the switch instructions. Hot is the wire that goes to the wire nutted bundle of blacks. Switched hot is the black wire (it doesn't have to be black, but it is in your particular case) going from the switch out of the box all by itself. If your switch instructions refer instead to line and load, line == hot and load == switched hot.