"Control group" is not one thing. Most strong projects use at least two.
Negative control — receives no treatment. Establishes the baseline. Nearly every experiment needs one.
Positive control — receives something already known to produce the effect. This is the one students skip, and it is the one that saves projects: if your positive control shows nothing, your assay is broken, and you have learned that before concluding "no effect".
Vehicle control — receives the carrier without the active ingredient. If you dissolved your compound in ethanol, the vehicle control gets the same ethanol at the same concentration.
Sham control — receives the procedure without the substance: the same handling, the same injection motion, the same time out of the incubator. Handling stress is real and it is a confound.
A useful check: for every step in your treatment procedure, ask "does my control group also experience this?" Anything that only happens to one group and is not your independent variable is a confound.