If there is no mass shown at the contact details this means the contact is not light up by active sensors. This means one of four things:
1) You're sensors are not switched on (all ships begin with active sensors switched off by default. Passive sensors are always on, which is why you can see the thermal signature of the target)
2) The enemy is outside active sensor range
3) Your sensors do not have proper resolution to detect the target at a given range
4) Your sensors are disrupted because of transit effects.
From what I can see from your screenshot 2) and 3) won't apply. The range is very short, so no matter your resolution you should be able to see him. For that matter, judging by the thermal signature the enemy is pretty large, so your standard anti-ship sensors should be able to detect is through it's entire range.
1) is always an option, especially for new players, but I think what happened here is that your sensors are disrupted because of transit effects. So here's a quick explanation.
After performing a jump, all electronic systems (which means active sensors and fire controls) are disrupted for a random amount of time. If you have performed "standard transit" this can easily last two whole minutes, maybe even a little longer, although I think the longest disruption I observed was about 100 seconds. If you have performed "squadron transit", which requires a jump capable vessel and limits the number of vessels that can move through a jump point to the "maximum squadron jump" property of the jump engine, then you can limit the amount of time your electronics stay disrupted to as little as twenty seconds. But you will always get at least some disruption.
Hope that helps.
